England and Wales High Court (Patents Court) Decisions

HC 1999 No. 02754

Mr David Kitchin QC and Mr Guy Burkill instructed by Messrs Olswang for the Claimant
Christopher Floyd QC and Mr Mark Vanhegan instructed by Messrs Weightmans for the Defendant

1 This is an action for infringement of European Patent (UK) no. 0042 723 ("the Patent") which is owned by the plaintiff, an English company, Dyson Appliances Ltd ("Dyson"). The Patent which is entitled "Vacuum Cleaning Appliance", was originally owned by another company Rotork Appliances Ltd, from whom the present proprietors derive title. The sole inventor is Mr James Dyson who gave evidence in this case on Dyson's behalf. The relief prayed for is of the kind usually sought in patent infringement actions, that is for an injunction, delivery up and either an enquiry as to damages or an account of profits.

2 The defendants are another English company, whose name over the years has achieved the status in this country of household, even eponymous association with domestic vacuum cleaning appliances. I shall refer to the defendants as "Hoover". Hoover deny infringement and have attacked the validity of the Patent by counterclaim for its revocation (and for declaratory relief) on the statutory grounds of lack of novelty, obviousness and insufficiency.

3 Hoover also counterclaimed in respect of alleged groundless threats by Dyson but this was compromised shortly before trial and is to be dealt with as an agreed item in the Order which I shall make at the conclusion of this judgment.

4 This dispute concerns vacuum cleaning appliances which depend upon use being made of cyclonic separation to deposit dirt from dirt- laden air which has been sucked through the apparatus from a location which requires cleaning. Such appliances do not require the user either to empty or to replace dust-collecting bags which prior to the date of the Patent, were habitually used in vacuum cleaners. Vacuum cleaners of the kind in issue were therefore referred to at trial as 'bagless' vacuum cleaners.

5 The product which is alleged to infringe is the Hoover Triple Vortex ("HTV"), Hoover's first venture into the field of bagless vacuum cleaners. Claims 1,2,4 and 5 of the Patent are alleged to be infringed and these claims are relied upon by Dyson as having independent validity. I have inspected and used an example of the alleged infringement.

6. I have also inspected and used Dyson's DC 01 appliance, which is I believe, one of its current commercial models. The Dyson DC 01 is an upright appliance which appears to me to be made in accordance with the Patent; it is prominently labelled 'Say goodbye to the bag'. I would add that both these appliances were made available to me for evaluation by the parties' solicitors. Both appliances are manufactured using certain transparent components which enable the user to view the cyclonic separators in action and note the accumulation of dirt.

7 There was no dispute that the earliest priority date of the patent is 19 June 1980. For quite some time before then, two kinds of domestic and industrial vacuum cleaner were on the market; both made use of bags. There were those that blew dirt-laden air into a cloth bag by means of a fan and electric motor; such bags had simply to be emptied from time to time. Then, more commonly, there were those that sucked dirt-laden air through a paper bag, the so-called bag filters. When the bag became full of dirt, it was thrown away and a new bag fitted. There were a number of domestic and commercial vacuum cleaners of these kinds on the market, as may be seen from Exhibit JD- 3 to Mr Dyson's Witness Statement. This is an extract from an Argos catalogue for 1980/81 which shows the large number of vacuum cleaners making use of bags (made both by Hoover and other well-known names in the domestic appliance field), which were available at that time. From this, I infer that the vacuum cleaner industry is competitive. Dyson stress that prior to the date of the Patent, and in spite of the competitiveness, nobody in the field of vacuum cleaner manufacture, domestic or industrial, had commercially proposed or sold a bagless vacuum cleaner. Based on the evidence, which is almost exclusively the unchallenged evidence of Mr Dyson himself, Dyson's proposition seems to be correct. In fact, Dyson go further, submitting that at the priority date of the Patent there was a palpable 'mindset' in the industry on the subject of bags, to the extent that dirt collection in a vacuum cleaner by any other means was regarded as heresy or folly – or both. Going by the sketchy evidence which I have as to the state of the art- a topic to which I shall return- that too seems to be correct.

8 There is of course, profit for vacuum cleaner manufacturers in the sale of paper dust-collection bags, since these usually fit only their own appliances. At trial, there was some dispute as to the approximate monetary value of this after-sale market but it is not one which I need resolve. I accept as have Hoover, the evidence of Prof Syred, Dyson's expert, that:

9 Dyson submits that the way the inventor came to make the device subject of the Patent is relevant at least to the issues of common general knowledge and obviousness, both of which fall to be considered later in this judgment. It is said that not only did Mr Dyson break new ground in the vacuum cleaner field by bringing to it cyclone technology but that the device claimed was the result of anti-intuitive thinking within the applied cyclone field itself, possibly even the result of serendipity. I have already referred to Mr Dyson's Witness Statement, the contents of which were largely unchallenged.

10 Mr Dyson's Witness Statement describes how he came to make the device the subject of the Patent and of the difficulties he encountered in realising its commercialisation. This is a personal odyssey which makes fluent reading. I shall consider its relevance (if any) to what I have to decide at a later stage.

11 Mr Dyson appears to be a man of practical ability, persistence and business acumen. Prior to becoming involved in the field of domestic vacuum cleaners, he had some experience with commercially launching a number of very different products such as the Sea Truck and the Ballbarrow. During the period he was involved with the Ballbarrow, he first came to know something about cyclones which he used at the Ballbarrow factory to purify air laden with epoxy paint.

12 The next event of importance occurred at home in October 1978. Among his domestic chores, Mr Dyson it seems had the task of doing the vacuum cleaning. The Dysons possessed a Hoover Junior for this purpose which made use of a cloth bag. Having been struck by its inefficiency, Mr Dyson bought another vacuum cleaner, this time of the filter bag type. After an initial period of adequate performance, it too proved to be less than satisfactory. His vexation at this state of affairs may then have led him to think of his recent experience with factory cyclones as a means of ridding air of unwanted material. Whatever the reason, he amputated the bag from his Hoover Junior and replaced it with a miniature cardboard cyclone which he had made airtight with tape. He tried it out and it indeed collected dirt. He had made a crude bagless vacuum cleaner working on a cyclonic principle.

13 That however was merely the first step. As a result of this experiment, Mr Dyson began to develop a technical interest in improving the dirt collecting performance of vacuum cleaners by this means. He bought some textbooks and went shopping in Bath to investigate currently available models of vacuum cleaner. As a result, he concluded that the technology had not greatly changed since Booth's original invention for the British Vacuum Cleaner Company in 1902: all the appliances of which he became aware used various kinds of bags. He next constructed some experimental cyclonic vacuum cleaners at home and in April 1979, made his first patent application based on this work. In due course, this application became EP no 0018 197. It is mentioned in two paragraphs of the Patent and was published just after the priority date of the Patent. I shall refer to it as 'Dyson I'.

14 Dyson I describes a vacuum cleaner having two identical inverted cyclones in series, the idea being that the sequential addition of a second cyclone to back up the first would afford the user 'added efficiency'. It is to be noted that both cyclones in this device comprised a conical portion. Mr Dyson demonstrated this prototype to AEC, a major vacuum cleaner manufacturer, but the demonstration was a failure. Though this machine worked satisfactorily in extracting fine dust, Mr Dyson was not satisfied with its ability to cope with the variety of materials which are commonly to be found in household dirt. The double cyclones still allowed other common domestic debris such as carpet fluff, thread, paper shreds, dog hairs and suchlike to pass out of the 'clean' air exit of the appliance. There was also some 'hang-up' of material within the cylinders themselves ie material which kept spinning about without being deposited. As Mr Dyson put it: " Other materials had problems of their own". Mr Dyson's next move was therefore to experiment with a number of modifications to these double cyclones. He changed their capacity relative to each other, he put helical tracks within them and so on. But

1. The details are in the evidence.

15 Still trying to solve these problems, Mr Dyson decided to modify a commercial 'tank' type vacuum cleaner. This sort of product is not strictly speaking a domestic vacuum cleaner but rather is intended for use in larger premises. It is what has been referred to elsewhere in the evidence as an 'industrial bag cleaner'. It consists of a metal drum with a motor incorporated into the lid for creating the vacuum and is provided with a large bag to collect the dust. Mr Dyson thought that if he placed a small frusto-conical cyclone within the 'tank' immediately under (and connected to) the motor under the lid, and also cut out the base of the collecting tank, replacing it from below with a larger frusto-conical section, he might be able to combine more efficient two-stage cyclonic separation with compactness of the appliance. He wanted in fact to 'nest' the two cyclones, one within the other. He accordingly fitted a tangential pipe entry for the incoming dirty air into the 'tank' wall and a short internal pipe from the 'tank' into the smaller conical cyclone for second stage separation. It was at this moment that fortune again visited Mr Dyson. Before the final modification which he had in mind,

16 The result came as total surprise to Mr Dyson. The 'tank' acted as a cylindrical cyclone removing the larger material As Mr Dyson put it:

And

3. Then, after further experimentation, he built a prototype and applied for the Patent.

17 The remainder of Mr Dyson's Witness Statement is directed to his initially fruitless efforts to interest the then domestic vacuum cleaner industry both here and abroad in licencing the Patent. This evidence was submitted to be relevant to certain questions which fall to be considered later and I shall return to it. Mr Dyson was the only non-expert witness to be called. I should therefore record at this juncture that though the time he spent in the witness box was short, I found him to be a fair and careful witness.

18 The Claimants called Professor Nicholas Syred on their behalf. Professor Syred has been Professor of Mechanical Engineering at Cardiff University since 1992. His evidence was that although his career has been spent principally within academic institutions he has often acted as a consultant to industry and that he has had considerable experience in the field of cyclones and swirl flow devices. He is in fact co-author of a number of books including a work entitled "Swirl Flows" (1984) to which reference was made in the evidence.

19 The Defendants called Professor R.W.K. Allen on their behalf. Professor Allen is Head of the Dept. of Chemical and Process Engineering at Sheffield University, a position which he has held since 1995. From 1976 up to his arrival at Sheffield, Professor Allen was employed by the then UKAEA at Harwell where he became involved in the study of inter alia gas cleaning technology, including cyclone design. He had a number of patents to his name.

20 Both experts were undoubtedly distinguished persons in their fields, respectively mechanical engineering and chemical and process engineering. They were witnesses who were called essentially to give evidence about cyclones, cyclone technology and the parties' experiments and were in my assessment, witnesses of complete candour. However it is important to recognise that at the priority date of the Patent (ie June 1980), neither expert was in any way interested in the field of vacuum cleaners (as opposed to industrial gas purification plant, process equipment for handling particulate materials etc). Indeed neither became so interested until much later. On occasion, under cross-examination, both witnesses had therefore to rely on past experience in their respective fields alone in answering a number of the questions which were put to them.

21 Looking ahead to the issue of who is to be regarded as the relevant person skilled in the art (Part I, para 30), it was common ground that neither expert could approximate to being such a person. As Prof Syred put it:

4. For the record, I do not consider that Mr Dyson can be regarded as a person skilled in the art either.

22 Before considering the Patent, it may be useful to make some introductory observations regarding separation cyclones and how they work, since the operation of these devices is a major issue in the present dispute. Like most technical matters, cyclone engineering and a fortiori, gas cleaning, is a subject which is capable of prolix technical elaboration and it may be necessary to examine the topic in more detail when I consider such matters as common general knowledge, infringement, obviousness, the parties' experiments and the experts' evidence.

23 Cyclones have been in use for many years before the priority date for a variety of industrial applications: in general chemical and food processing (where eg powders are collected after cyclonic separation), as air purifiers (eg as static dust collectors in factories and in power plants), as product agglomerators, in the petroleum industry for separating water/oil mixtures, and so on. Cyclones come in various forms and sizes and particular applications may demand modified apparatus. Cyclones have the advantage of easy construction, operational efficiency and low maintenance. This case is concerned with the use of cyclones to separate dirt from dirt-laden air. For this application, a cone-shaped cyclone is normally used. Such cyclones are called 'reverse flow' cyclones, because the air exits at the same end of the cyclone as the dirty air inlet, thus involving a reversal of the axial flow of air in the unit.

24 The main features of this cyclone separator are shown in Annex 1 to this judgment. Dirt-laden air enters the cyclone under negative pressure via a tangential inlet duct situated on the circumference of a short cylindrical portion of the apparatus which thereby converts a tangential inlet flow into rotational motion. The gas spirals around and (there being a pressure drop across the device) down the cylindrical portion until it reaches a lower conical or tapered portion of the cyclone. This is where separation of dust or particulate matter takes place by exploitation of the principle of the conservation of angular momentum. The gas is forced to spiral ever inwards towards the axis of the conical section with increasing tangential velocity. The centrifugal force arising from the rotation of the gas becomes greater as the internal radius of the conical portion decreases, throwing the suspended dirt or particles outward to the region of the sloping cyclone wall with increasing force. The cone thus focuses the dirt downwards and along the sloping internal surface of the wall of the conical portion and thence via a collar (or suchlike) into a collecting hopper below the lower end of the cone. The cleaned gas spirals up in the opposite direction to the dirty air into what is called the vortex finder (or outlet) and thence out of the cyclone.

F The Patent

25 The descriptive part of the Patent is short and is not difficult to understand. It is concerned with vacuum cleaning appliances which exploit two kinds of cyclone unit which operate one after the other in order to separate dirt from dirt-laden air which has been sucked up from a dirty location by a motor-driven fan. The appliance is said to have the advantage of being 'bagless'. Dust separation is achieved first in a "lower efficiency" cyclone unit and following it, a " high efficiency" cyclone unit. The reader is told first of all about the high efficiency unit by direct reference to the type of cyclone separator described in Dyson I which, as the narrative of Col 1 makes clear, is to be read into the Patent. I shall therefore first consider what Dyson I teaches concerning these high efficiency cyclones.

26 The introductory paragraphs of the Patent read as follows:

5. The introductory paragraph of Dyson I itself reads as follows:

6. In fact, both the Dyson I appliance and that described in the Patent are primarily intended for domestic use. As already noted, Dyson I describes two cyclone units operating in series to rid dirt-laden air of its dirt. The second cyclone unit is 'located downstream' of the first and 'provides a fine or secondary cleaning unit' (Dyson I, page 10). The cyclones of Dyson I are examples of the kind of conical separation cyclone which I have described in general terms in Part E ('Separation Cyclones'). It is to be noted that Dyson I refers to these cyclones (22) and (23), as being 'substantially frusto-conical in shape' (Dyson I, p 8, ll 19-21). Referring to Dyson I, the Patent also states that

7. Such 'tapered cyclone units' are thereafter referred to in the Patent as 'high efficiency' cyclones, efficiency being used in relation to the ability of the unit to deposit fine dust particles (Col 1, ll. 33-36).

27 The narrative then refers to a single item of prior art (US patent no. 3,425,192 to Davis) to which both sides referred. Davis proposes a two-stage, in series, vacuum cleaning system using high-efficiency conical cyclones.

8. The first stage of Davis is a single conical cyclone (11): Davis, Col 1,ll 34-35. Downstream of the first stage, the second stage (12) is a collection of six such cyclones arranged to work as a single unit in parallel, being an example of what was called in this case, a 'multicyclone unit'. The cleaned air from the first stage passes therefore to the second stage where it divides into six airstreans all of which are thereafter subjected to a secondary cleaning process. The Patent thereafter identifies the problem for which it provides a solution:

The solution proposed is stated thus (Col 1,ll 56-63):

9. The lower efficiency unit is not capable of dealing effectively with the finest particles (ie particles of 50µ or under) but carries out a primary cleaning operation, removing 'all but some of these finer particles' (elsewhere'the majority of the dust particles') from the dirty air. The high efficiency unit 'with the taper'

10. This operation was referred to by Dyson in argument (inaccurately, as Hoover say), as "an initial wipe" followed by a "final polish". In a preferred embodiment (which is the only embodiment used to describe the invention), the high efficiency cyclone unit is placed within the lower efficiency cyclone unit thereby securing a compact appliance which is more suited to domestic cleaning. I have referred to this as being a 'nesting' configuration. I have annexed to this judgment (Annex II), Figs 1 and 2 of the Patent which illustrates this preferred embodiment. The way in which this device works is described from Col 2 line 25-Col 3 line 44. The cylindrical casing of the lower efficiency unit is item 13. The high efficiency cyclone unit:

11. The narrative then explains how the dust is collected and removed, the coarser dirt being shown at 'A' and the finer particles being shown at "B" in the Fig 2 drawing. Differently marked arrows in the drawing show the progress of the airflow through the device as it gets rid of dust.

12. G The Inventive Concept

28 It is convenient that I deal with this now. The identification of the inventive concept monopolised in the Patent is a task which assumes particular significance when the questions of 'variants', obviousness and insufficiency are examined. Mr Floyd QC for Hoover warned me against attempting epitome if the broadest claim of the Patent is clear. There is undoubtedly authority to support his proposition. In my experience however, the identification of the inventive concept is an habitual feature of patent infringement actions where the subject matter is amenable to straightforward epitome. In addition, there is recent Court of Appeal authority to support its utility in the assessment of obviousness, where appropriate: Glaverbel v British Coal [1995] RPC 255 at 278 and Union Carbide v BP [1999] RPC 409 at 424. Moreover, Dyson have proposed a summary of the inventive concept in their argument and I have therefore to deal with the issue in any event. In my judgment a simple statement of the inventive concept of the Patent can usefully be made in this case, and it is this:

13. I would however stress that this statement is not intended to be a substitute for claim 1.

H The Claims

29 There are five claims altogether, all of which are dependent upon claim 1. At trial, claim 1 was divided into the following integers for convenience, the words in contention being in italics:

Claim 2 incorporates the highest efficiency unit within a 'cylindrical' low efficiency cyclone- the 'nesting' configuration. It is the appliance of claim 1 additionally characterised by:

Claim 4 relates to an upright appliance on wheels having a cleaning head with a rotatable brush (4) at the dirty air inlet.

Claim 5 relates to the use of a belt from the motor of the fan to drive the brush (4).

14. None of the claims contain any technical words or ordinary English words which are used in a special sense.

30 A patent is addressed to persons who are likely to have a practical interest in its subject matter or have to act on the directions given in it for its implementation. The nature of the addressee must therefore depend upon what view is taken as to the proper scope of the subject matter of the patent in question. Furthermore, the degree of qualification of the notional addressee within that subject matter will depend upon the degree of technical sophistication which the subject matter demands. Both parties agreed that the addressee is deemed to be unimaginative and uninventive. Both parties were also agreed that the addressee could if necessary, be a notional, multi-disciplinary 'team'. What then is the field in which the notional addressee in this case is to be found and what are his attributes? The parties were not entirely ad idem as to the identity of this 'person' (or team) or its attributes.

31 The Patent is entitled "Vacuum Cleaning Appliance". Dyson initially submitted that the addressee was to be regarded as a general engineer working in the vacuum cleaner industry with " a practical turn of mind" and probably of post-graduate or HND level. They stressed the references in the Patent to the domestic usage of the appliances described (and exclusively claimed, in the case of claims 4 and 5) and at one stage, suggested that the addressee was to be found in the domestic vacuum cleaner manufacturing field.

32 When the experts' reports were exchanged, Professor Allen visualised Hoover's skilled addressee as follows:

15. Professor Allen's view at the time seems to have been influenced by his belief that the Patent provided few working details on how to make either the cyclones described or even a 'typical' vacuum cleaner. However, it was not in fact argued that the construction of the appliance described in the Patent would present any serious problems even to a modestly qualified team of vacuum cleaner engineers without specialist cyclone expertise. By the close of proceedings, Hoover's position seemed to me to have changed somewhat. They now proposed an addressee with the basic qualification of being an appropriately experienced 'designer and manufacturer of vacuum cleaning appliances' - though, since the Patent was not limited to domestic vacuum cleaning appliances, one who had a broader base than in the domestic field alone. That basic qualification seems to me to be entirely reasonable. Beyond that, Hoover would also attribute to the addressee some detailed knowledge of the theory and working of various kinds of cyclone. By closing submissions, Dyson's position had also shifted. They accepted that the addressee should be imputed with some limited knowledge of large-scale industrial cyclones of the conical type and how they worked. In the end, I think that the difference between the parties became a matter of degree on the question of how much 'cyclone expertise' could properly be imputed to the addressee: 'The Syred man' and 'the Allen man' as Mr Floyd QC labelled them.

33 By the standard of many patent infringement actions which have been before the courts in recent years, this does not involve a technically sophisticated subject matter. In Mr Kitchin's words, it is not 'rocket science'. This is a patent in the vacuum cleaner field, both domestic and industrial. I therefore consider that the addressee should not be regarded as at all times including an engineer having inter alia specialist 'hands on' cyclone expertise - such as for example, a chemical engineer having experience in designing cyclones for gas scrubbing or the processing of particulate matter. I believe that Hoover's submission goes rather too far on this question.

34 The evidence has not proved to be of much assistance in getting a clear picture of the workings of what I believe to be the relevant field, that is, the vacuum cleaner industry alone, probably in a middle-sized company. Nevertheless, in my judgment, the notional addressee is to be regarded a graduate or HND engineer in either mechanical or electrical engineering (probably, a team of both) with a few years practical experience in the vacuum cleaner industry, the addressee's experience and expertise having a broader basis than that of domestic appliances only. Such a person would I believe, as part of his general engineering background also have some knowledge, latent or otherwise, of the workings of commercial cyclones - for example, of how separation occurred within a conical cyclone, even if such knowledge was what Prof. Syred called 'scant' (Report, para 16).

35 As I have mentioned, Prof Allen considered that at the priority date, a 'hands on' cyclone expert should be a permanent feature of the team. Whilst I accept that the notional addressee may properly be regarded as a team, I do not consider that in this case the ongoing presence of a cyclone expert need be invoked.

36 Hereafter, when I refer to the 'skilled man' or to the 'addressee', I intend the term to be understood as being a notional team having the approximate attributes outlined in this section.

16. J The Common General Knowledge

37 Establishing the common general knowledge of the art is a necessary exercise in a patent infringement action since in most cases it will have a bearing on construction, infringement and the proper assessment of the cited prior art. Hoover submit that whatever be the appropriate level of common general knowledge, the same level is applicable when each of these topics is adjudicated. I agree with that proposition. Dyson submitted in closing that even if the field of the addressee's knowledge encompasses both domestic and commercial appliances, the differences between these fields would make no difference with regard to common general knowledge. I also agree with that proposition.

38 The correct assessment of common general knowledge in patent cases has for some time been shaped by the classic judgment of Sachs LJ in General Tyre which I need not repeat. Dyson has also drawn my attention to the more recent judgment of the Court of Appeal in Beloit v Valmet particularly to the following:

39 The principal factual evidence of the state of the common general knowledge of the art was the opinions of the two experts, neither of whom it will be recalled, had anything to do with the vacuum cleaning industry either at the relevant time or for some time thereafter. I have made allowance for that in what follows. Hoover were of course major players in the vacuum cleaner industry long before 1980 but led no evidence as to what they or their competitors were doing or thinking at the time. This is seen by Dyson as a deliberate tactic which they say, should at least generate no indulgence in Hoover's favour on my part. I shall return to the point at a later stage. I must add that Mr Dyson's evidence as to what the industry was not doing (or thinking), albeit negative, may in my view be taken into account in the assessment of the state of the common general knowledge: see para. 44 below.

40 The common general knowledge which may in my judgment properly be attributed to the skilled addressee will first include a detailed knowledge of the working, construction and manufacture of vacuum cleaning appliances intended for domestic and industrial application at the priority date. This may I believe, reasonably include knowledge of the workings of the majority of those appliances which were commercially available both nationally and internationally, and whether made 'in-house' or by competitors. Within reason, it would include past models as well. In particular such knowledge would include:

41 On the evidence and with the qualification which follows, I do not consider that experience in commercial cyclone theory, design and technology can properly be considered to have been part of the common general knowledge of the person skilled in the art at the priority date. In the vacuum cleaning industry at the time, there was as Hoover put it, " an addiction to bag technology" and thus no reason even to think of using any kind of cyclone as a dust separator in vacuum cleaners. The skilled worker would therefore be most unlikely to have 'hands on' or working knowledge of the varieties or design of cyclones or their commercial application, let alone knowledge of the complex physics of gas flow within them. But that broad statement must be qualified. I have said that the addressee, would nonetheless have had what I have already referred to as latent general knowledge of commercial cyclones which he might have acquired for example, as a result of his graduate training in general mechanical engineering. This would be more than a layman's knowledge. I consider that he would have been well aware of the sort of device which I have described in Part E above, that is, the large industrial type of cone separator used for gas purification or product separation/collection and how it worked, even if he had no practical experience in designing one. As Prof Allen said in his Report (para 35):

42 He would therefore understand in general terms, how the reverse flow cyclone works and in particular, the function of the frusto-conical shape in the particle separation process. Cyclone design had not it seems much changed for some time before 1980 and as noted, this was the commonest design of cyclone at the time. He would have understood that in such devices, separation from a fluid medium takes place by exploitation of the conservation of angular momentum through the action of centrifugal forces on dirt-laden air, in the frusto-conical portion of the device.

43 The physics of fluid flow in cyclones can be extremely complex and prone to conflicting and speculative explanation. This is readily apparent from the expert evidence given at several points in this case. I do not consider any of this to be part of the common general knowledge of the addressee. Furthermore, in my judgment, it is most unlikely that the skilled worker would have been aware of certain other types of cyclone such as:

17. I shall consider these in a little more detail below.

44 I must briefly return to Mr Dyson's evidence which has an important negative bearing on the state of the common general knowledge. The latter part of Mr Dyson's evidence was concerned with events occurring after the filing date of the Patent. It seems that Mr Dyson sought to interest the vacuum cleaner industry (not just in the UK) in his cyclonic vacuum cleaners, first with Dyson I and later, with the device subject of the Patent. This happened just after the priority date. Mr Dyson's evidence shows that:

45 Dyson relied on this evidence for various purposes but principally so as to provide some insight into the state of the common general knowledge at the relevant time. In this connection, they also reminded me on several occasions that although Hoover were well placed to do so, they had adduced no direct evidence either in-house or otherwise on this topic. From all this, Dyson invited me to make a number of findings relating to various topics in issue. This is secondary evidence (in one case, hearsay as well) and I have accordingly approached it with caution. Nonetheless, at its lowest, it is consistent with an assertion that expertise in cyclone technology formed no part of the addressee's active repertoire of skill and knowledge or even thinking. They were it seems, truly 'addicted' to bag technology.

46 Though a number of textbooks on cyclone technology were referred to at various times during the trial, I do not consider that any of them formed part of the common general knowledge of the skilled worker at the priority date. Neither did any of the prior art citations.

47 There are some items of prior art in the cyclone field which at any rate at the beginning of the trial, Hoover asked me to treat as forming part of the common general knowledge and which I must therefore consider in more detail.

Cylindrical cyclones.

18. In my judgment, the skilled man would not have been aware of the existence of these devices. This was the view of Prof Syred. Prof Allen was also cross-examined about the use of cylindrical cyclones at the priority date and stated that they were used only for specialist purposes such as in combustors and in military engines, both being far from the field in question.

Cyclones mounted in series.

19. It is in my judgment most unlikely on the evidence that the man skilled in the art would have been aware that cyclones had been used in series for additively better material separation. Prof Allen suggested that the use of cyclones in series was well known at the priority date, such systems having been common practice from the 40's to the early 60's. This evidence had seriously to be qualified under cross-examination . However logical 'in series' operation may now seem, on the evidence, I do not believe that in series cyclone operation was part of the common general knowledge even of a person having practical experience with cyclones.

20. Hoover have found a reference to the 'nesting' of cyclones in a 1966 textbook by Coulson & Richardson entitled Chemical Engineering to support the proposition that in 1980 " the idea of a cyclone within a cyclone was a well known piece of technology". It is on pages 553-554 of the text (7B/5 pp13-14) and is a short passage with a diagram in a substantial two volume work covering a very large number of topics of interest to chemical engineers, none of which have anything to do with cyclones. In cross-examination, Prof Allen stated that this was indeed the only reference of which he was aware in any material relied on by Hoover wherein reference was made to cyclone 'nesting'. Prof Syred was also cross-examined on this passage (3/326-327). He was of the view that cyclone 'nesting' was not at all well known even to him in 1980. It was not even taught on specialist cyclone courses. He also pointed out that the reference upon which Hoover relied, related to a crude multicell arrangement of cyclones (only) " in a big box". In my judgment, the notion of cyclone 'nesting' was not only remote from any knowledge which the notional addressee was likely to have possessed in 1980, but seemed even to be a rather exotic item to the experienced cyclone specialist. I have no hesitation in rejecting it as an item of common general knowledge.

48 I was addressed by both parties on the principles of law to be adopted in construing patent specifications, but in the end I detected no significant difference between them. In taking into account the following considerations, I have nevertheless tried to avoid an over rigid adherence to any canons of construction.

49 In Part H above, I set out claim 1, dividing it into integers, and putting into italics words and phrases which require consideration under this section. The letters in brackets which are given by each of the following sub-topics, refer to the division of integers in claim 1 referred to above.

50 The use of appliances which rely on a vacuum for their operation, are common. Such devices are used in industry, scientific research, in hospitals, etc and in the home. The word 'cleaning' in the above phrase is therefore the determinative word. This word too is capable of fairly broad application so it must be examined more closely.

51 Dyson initially submitted that the term should be given a meaning confined virtually to domestic vacuum cleaners. In this connection they have pointed to the fact that the Patent (and the Dyson I patent) is indeed concerned principally with domestic appliances. No other type of vacuum cleaner is either described, exemplified or illustrated. The problem which the Patent is said to solve arose from the inability of the Dyson I device to cope with

21. Dyson point to the US Davis patent mentioned in the Patent as prior art which is entitled 'Vacuum Cleaning System' ('the usual hose attached to a conventional suction tool') and also rely on extracts from standard dictionaries to support their submission that the phrase has acquired a limited secondary meaning that alters the broad literal meaning of the three words of which the phrase is comprised. They accept that "appliance" can refer either to a fixed or mobile piece of equipment.

52 Hoover favours a broader construction which includes sundry domestic, industrial and commercial appliances, fixed or mobile and whether using a cleaner/pick-up head or not. They point out that it is not until claims 4 and 5 that one finds devices which fairly read onto domestic cleaning appliances alone. Claim 1 and 2 they say, must be broader in this respect than the later claims and should include 'any device relying on suction which is capable of separating and depositing fine dust from a gas'. That would for example include 'stand alone' industrial gas separators, cleaners and scrubbers and similar devices. They have prayed in aid usage of the word 'cleaner' in items of prior art upon which they rely. They have also cited some evidence of varied usage of the word 'cleaner' given by Prof Syred in cross-examination.

53 I have not found Hoover's reference to the use of the word 'cleaner' in their cited US documents to be of assistance and have disregarded it. It is not I believe, correct to construe ordinary English words in a patent by reference to usage of the same word in foreign documents of an earlier date. Similarly, I have disregarded the evidence of Prof Syred as to how certain devices known to him were described at unspecified times. 'Cleaning' is in the end, an ordinary English word.

54 To clean something, a surface or location say, presupposes that it is dirty. For this one may use for example, a 'vacuum cleaning appliance'. One must therefore understand the teaching of the Patent in a cleaning context. One asks, what is the principal purpose of the appliance described therein? It is to clean. Next, what is it intended to clean? There are logically three possibilities : a dirty surface of some kind, particle-laden air, as in a scrubber or an air purifier - or possibly, both.

55 In my judgment, the device described and claimed in the Patent is intended to clean a variety of surfaces or locations which have become dirty or covered with unwanted particles. It is not intended to function as an air purifier as such or as a dust extractor, though having performed its primary surface cleaning function, provision must obviously be made to retain the dirt within the device until the user wishes to discard it. The user of the device proposed in the Patent has only a secondary interest in the cleanliness of the air which is finally expelled from the device. Still less is the phrase to be understood to comprise devices such as a particle collector or particle agglomerator. The user of the appliance described and claimed in the Patent places no value whatever in what is extracted from the dirt-laden air; it is discarded. Thus, though the appliance claimed 'cleans' first a surface and thereafter the dirt-laden air sucked from that surface so as to rid it of unwanted dirt, its primary purpose is simply to clean surfaces.

56 However I agree with Hoover that the device claimed is not intended only for use in a domestic environment. Claims 1 and 2 are apt to cover a vacuum cleaning appliance for use in industrial and commercial establishments or in public buildings, schools or churches etc where machines of more robust and capacious construction may be required.

57 This construction of the opening words of claim 1 is not only in harmony with the rest of the specification but also with the dictionary definitions proposed by Dyson.

(b), (e) and (f) 'Cyclone unit(s)'

58 This phrase occurs three times in claim 1 and I shall therefore consider it separately. It is common ground that the words 'cyclone' and 'cyclone unit' are used interchangeably in the body and claims of the Patent. A cyclone unit is on its face, intended to encompass more than just the separator vessel itself (or a part thereof) wherein dirt is physically separated from dirt-laden air. Take for example, the 'high efficiency cyclone unit' described in the Patent. This ' cyclone unit' properly includes an 'air entry', that is, the inlet pipe (19), the tangential entry port (18) and the cylindrical upper portion of the separator (un-numbered). In addition, there is a frusto-conical 'body portion' (15) together with the dependent cylindrical portion (16): see Col 2 lines 60-65. All of these are parts of the 'cyclone unit'.

59 Furthermore, it was submitted by Dyson (and I accept) that:

22. Thus a 'cyclone unit' may contain within it other cyclones and/or ancillary apparatus co-operating with a cyclone or cyclones.

60 Having come to this view on the basis of the contextual meaning of ordinary English words, I thought it prudent to check the evidence to see whether this construction conflicts with technical reality, as seen by the experts. Not only is there no such conflict but the evidence of both experts amply supports this construction. Thus, in relation to para 59(a) above, there was evidence from both experts about a device called a 'multicyclone' or 'multicell cyclone':

23. This side arm cyclone itself consists of three parts: a cylindrical portion, a frusto-conical portion (allegedly two working in tandem) and a side arm air exit : see diagram on 7C/14 and cross-examination 3/414-418 to 4 /424-428. Of this, Prof Allen said:

24. As the experts see it, a multicyclone can thus properly include in one vessel, an array of cylindrical separators in parallel, an array of frusto-conical separators in parallel and a hybrid pair of cyclone separators, one being cylindrical the other frusto-conical in series.

61 Finally, this view is consistent with the function of the second stage separator multicyclone separator in Davis; it is a cyclone unit: see para 27 above.

62 I have dealt with 'cyclone units'. Efficiency relates not to cyclone geometry as such but to the unit's ability for whatever reason to collect fine dust and very fine particles. Moreover, it is clear that all these units must be constructed so as to be capable of actually depositing fine dust – and some may be more efficient at so doing than others. Dyson stressed the importance of the word "depositing", in the sense of first cyclonically concentrating and separating and thereafter taking the dust out of the cyclone system altogether, to a particular place – or having the capability of so doing. A cyclone unit which did not (or was not designed to) actually deposit dust, was they argued, not a 'cyclone unit' within the claims. I agree. I would add that the skilled worker would recognise the ability to deposit particulate matter after separation as being part of the prescription for a conical separation cyclone.

63 The body of the patent itself provides the definition of 'higher efficiency': Col 1, ll. 25-37. From the technical point of view, neither expert seems to have had any difficulty understanding the phrase in just the manner proposed in the Patent: Syred Report, paras 84-87 and Allen Report, paras 53-54.

64 I shall consider these phrases together because purposive construction logically requires a simultaneous and uniform consideration of them; they all relate to the progress of the air flow as it passes through the appliance before being finally expelled. The correct meaning of these words is I consider, directly connected with the central purpose of the invention, which is expressed in Col 1, ll 63 to Col 2, ll 15 in words which I have already set out, in part:

25. I shall consider first the word 'sequentially'. An air flow is established through the machine by the motor and fan which starts from the dirty air inlet where there then exists a 'dirty air flow'. Thereafter, primary cleaning in the 'lower efficiency cyclone unit' leads to 'comparatively clean air' which is now free of larger particles. This air is next presented to the 'highest efficiency unit' for ridding it of 'the finest particles'. Finally, the cleaned air is expelled. That is the 'sequence'. 'Sequentially' in integer (e) means no more than that an operation substantially to rid the air of dirt is performed on an originally dirty air stream in the prescribed order.

65 'In series' is normally used in English in the sense of 'one after the other' or 'one thing following another'. This is of course a general usage and its contextual meaning must be assessed in the light of the purpose to be served by (in this case) the requirement that the cyclone units are to be arranged 'in series'. It is clear that from the functional point of view, the 'in series connection' of claim 1 requires that all the 'comparatively clean air' from the lower efficiency unit is delivered to the 'highest efficiency unit'. Dyson's submission was simply that that which is received by the latter must have been pre-cleaned. Hoover submitted that an important qualification had to be made to this in that all the cleaned air from the low efficiency unit should pass directly into the higher efficiency unit. In connection with this phrase, I was addressed by both parties on the analogy of electrical circuits (such as resistors) arranged 'in series' and 'in parallel' (which would no doubt have been familiar to the addressee). In my judgment however, the proper construction of the phrase 'in series' in context does not import the requirement that the output from the low efficiency unit must flow directly into the higher efficiency unit. The 'connection' does not have to be direct.

66 I have also checked the references to the use of the phrase 'in series' in the body of Patent for consistency with the above finding. The first is to Dyson I and I have already considered that: see para 27. The second is to be found in relation to the cited prior art patent to Davis (Col 1, ll 37-43) which, as previously noted, discloses a single first stage frusto-conical type separator unit for coarse dust removal which delivers its entire output to a multicyclone unit of six of smaller separators in parallel to remove the 'finer dirt'. The system is described as 'having first and second stages through which air serially flows' (See Col 1, ll 30-31 of Davies). All of the air which is going into the high efficiency second stage unit has come from the first stage – which is upstream of it. Hoover's argument on 'in series' would require that the output of the first stage should go through each of the six cyclones within the multicyclone - one after the other. But that is not the way Davis works. On the contrary, in my judgment the way Davis works would be taken into account on this point by the skilled reader.

67 Finally, the 'lower efficiency cyclone unit' must be 'upstream' of the 'highest efficiency unit'. This word is used in a general sense. Having regard to the direction of gas flow, it must merely be nearer the dirty air inlet than the high efficiency unit.

68 First, the parties were divided over what requires to be construed within this integer. Dyson submit that the entire integer falls to be construed as a unity whereas Hoover identify the task of construction as stopping after the part number '(15)'. I consider that Dyson's submission is correct since, though the integer would make good sense both as a matter of English and from the technical point of view without the further phrase 'tapered away from its entry', I cannot treat it 'as struck out if it does not appear to make any difference to the inventive concept': see STEP v Empson , (supra).

69 Let me first however consider the adjective 'frusto-conical'. This qualifies the noun 'part', which in normal usage describes the section of a cone between planes. This requires no further elaboration; purely from the point of view of geometry, straight converging sides are always required for a section to be frusto-conical. This is the only 'primary, literal or acontextual meaning' of the adjective. However, with an eye to the Protocol, the context must next be considered.

70 First, the Patent is silent as to either the dimensions (diameter and height) or the 'conicity' (ie angle) of this 'part'. One can envisage a range of possible cyclones all having frusto-conical sections, ranging from the steep to the scarcely perceptible. If this had been of importance to the efficacy of fine particle separation one would have expected relevant detail to have been given eg at least for the half angle of the convergence of the cone. As matters stand, the skilled reader would in my judgment, understand that he could choose with confidence any frusto-conical profile (in the 'geometric' sense) and that it would

26. He might therefore need to adjust the diameter, height and half angle of convergence to achieve the optimum result. Fig 2 in the Patent would thus be understood merely to be exemplary of a number of possible frusto-conical configurations for a conical separator all of which worked in the same way.

71 The parties accept this, but Dyson went further. They contended that 'substantially frusto-conical is enough' having regard to both context and to the function to be performed by the frusto-conical shape viz to maintain or increase the rate of swirl of the dust-laden air as it proceeds down the cone so that in accordance with the conservation of angular momentum, the device could productively deposit the fine dust. They point to the use of the word 'taper' or 'tapered' alone elsewhere in the Patent to describe the high efficiency unit and say that that is an intentionally inexact qualification.

72 Hoover on the other hand argued for the strict geometrical (ie literal) interpretation of the adjective 'frusto-conical' throughout the Patent. They use the reference to 'substantially frusto-conical' in the Dyson I specification, to highlight the absence of the word 'substantially' in this connection anywhere else in the Patent, suggesting that the omission had been deliberate and that the contrast would be noticed by the skilled addressee. They indeed accept that in Dyson I, the composite entity may accurately be characterised as being 'substantially frusto-conical' but argued that the words 'a body of substantially frusto-conical shape etc' at Col 1, line 27 of the Patent, describes the general shape of the entire separator units referred to in Dyson I (that is, so as to include the usual cylindrical lead-in portion), rather than to the frusto-conical part alone. They also pointed to the fact that Dyson I was not put to Professor Allen in cross-examination.

73 I have held that the skilled addressee as part of common general knowledge would be well aware of separation cyclones having a 'frusto-conical part' and of the role that this part plays in the way such devices work as separators . This knowledge, coupled with the direction to read Dyson I into the Patent, tends in my judgment to the opposite conclusion to that proposed by Hoover. First, I do not believe that Hoover are justified in drawing the conclusion they wish from the use of the words 'body of substantially frusto-conical shape' in the 'Dyson I' passage in the Patent (Col 1, ll 25-31). In that passage, the Patent says this of the Dyson I cyclones:

27. That usage is of course carried through to the claims. Furthermore, the teaching of Dyson I is I believe, clear without cross-examination. The high efficiency cyclone unit required for use in the Patent is the very cyclone of Dyson I ('such tapered cyclone units') which is itself described in Dyson I as being 'substantially frusto-conical in shape' (see Dyson I, p. 8 and the drawing). Furthermore, it works in the same way: Dyson I, top of p. 13. The word 'substantially' in Dyson I in my judgment qualifies the 'working' or 'business' portion of the cyclone. There is thus no requirement that this part of the separation unit must be a geometrically precise frusto-conical in either patent; but it must be 'tapered'. That is why it separates the dirt.

74 The phrase 'tapered away from its entry', logically applies to all frusto-conical separators, so its presence must have some further qualificative significance. In contrast to 'frusto-conical', 'taper' is, as Dyson say, not an exact word which requires a precise geometric configuration. They argue that it signifies a shape which gradually and visibly becomes narrower towards one end. In my judgment, the skilled reader would consider that the presence of the phrase 'tapered away from its entry', would indicate that geometrically precise frusto-conical sides are not essential for the high efficiency conical separator. This would accord with his common general knowledge as to how such cyclones work.

75 I therefore reject Hoover's literal construction of this phrase; it requires geometric precision, carrying with it the invariable requirement for straight sides to the separating cone. Whilst such a strict literal construction may afford certainty to third parties, in my judgment in this case, it falls foul of the Protocol which also requires 'a fair protection for the patentee'. The phrase no doubt includes strict geometry but in context, it is enough if the portion in question is 'sufficiently frusto-conical to do its job'.

76 As on previous occasions, I have considered the cyclone experts' evidence to see if this finding conflicts with technical reality bearing in mind that the complexities of gas flow in such devices would be beyond the knowledge of the skilled man. According to Prof Syred (First Report, paras 89-90), there certainly is no conflict. Prof Allen's Report on the point (para 98) strays into infringement topics, but even then, is in broad agreement with Prof Syred till half way through the paragraph; departure from strict frusto-conical geometry may, so he says, even enhance the dirt-collecting efficiency of the separator. The remainder of his evidence is in my view, speculative touching on unknown and unpredictable effects of 'boundary layers'. He concludes by saying that none of these qualifications would have been predictable to even the cyclone expert in 1980. If that is so, how much less likely would it have been apparent to the skilled addressee. I shall return to the Professor's evidence on the effect of departure from true frusto-conical shape in the high efficiency separator when I consider infringement.

77 Before leaving this issue, I cannot help being struck by the parallel between the debate about the word 'vertical' in Catnic v Hill & Smith [1982] RPC 183 and 'frusto-conical' in this case. The following words of Lord Diplock are relevant (at p 224):

78 The purpose of this limitation is so as to achieve a more compact appliance by the incorporation of the high efficiency separator within the cylindrical lower efficiency unit: Col 2, lines 7-17. There can in theory be alternative locations: 'wholly within' and 'partly within', the former achieving no doubt the greater compactness. The body of the Patent is not concerned with this distinction though the drawings, Figs 1 and 2, show the top third of the highest efficiency unit protruding from the top of the lower efficiency unit. In my judgment, 'within' does not require that the highest efficiency cyclone unit should be located wholly within the lower efficiency unit.

79 In recent years, the law on this subject has been dominated by the decision of the House of Lords in Catnic v Hill & Smith (supra) and by Hoffmann J's guidance as to the proper approach to the question of 'variants' to a particular word or phrase in a claim, in Improver Corp v Remington Consumer Products [1990] FSR 181 at 192. My attention was drawn by both parties to the summary of the law on infringement which is to be found in Terrell (15^th Edition), paras 6.12 – 6.16. It was said that those passages could be adopted as a convenient epitome of current law and I propose to use them as a working blueprint for what follows, without attempting further re-statement. I would add that I was reminded by Mr Kitchin QC for Dyson that

80 In proving infringement, the onus remains on Dyson throughout. There is no shifting onus: see per Stuart-Smith LJ in ICI v Montedison [1995] RPC 449 at 473.

81 The vacuum cleaning appliance which is alleged to infringe, the HTV, operates without bags, making use of three cyclone separation units (which by agreement, have been designated SU1, SU2 and SU 3 respectively) to separate dust from the ambient air sucked in by the machine. It is not easy adequately to précis the mode of operation of the HTV. A Product Description of this machine was prepared for trial in accordance with CPR 49 EPD 9.2 (1). However, in relation to the functioning of one part, SU 2, it has not been possible for the parties to agree either the narrative or the relevant part of the drawing appended to it. Needless to say, this is where the greatest controversy in this part of the case arises. What I propose doing therefore is to summarise the parts of the description of the HTV about which there seems to be little or no controversy and then to quote verbatim from the Product Description itself when it comes to SU2 and SU3. This portion of the description specifically identifies the problem areas. A schematic diagram of this product is appended hereto, as Annex III.

82 Both sides performed experiments, full details of which are to be found in Bundle 1B. I shall return to these experiments later.

83 Very briefly, the HTV appliance is an upright vacuum cleaning appliance which sucks in air from a dirty surface by means of a fan and electric motor, thereby creating an initially dirty airflow. The device is equipped with a belt-driven rotating brush, flexible hose and associated passageway/ 'dirty air inlet' for channelling dust-laden air into its base. The dirty airstream is then worked upon by the three cyclones, one after the other, SU1, SU2 and SU3, all of the air exiting SU1 going first into SU2. Substantially clean air is exhausted from the top of SU2. As it is put in the Product Description:

29. Provision is made for the retention of dust/debris which has been removed from the airstream. This is deposited in two transparent plastic collection canisters for disposal.

84 The function of the three cyclones is described by Hoover in its commercial literature as follows:

85 Prof Allen agreed that all the airstream presented to SU3 has first passed through (that is, has been pre-cleaned) by SU1. It was also agreed that in normal use,

86 SU1

87 SU2 and SU3

34. It is in this area of operation of the HTV that there is substantial controversy. I shall therefore quote verbatim from the Product Description in order to describe what the parties have agreed happens within these units – and where they disagree as well.

88 Dyson's case on infringement was originally set out in a letter from their solicitors dated 16 September 1999 in response to Hoover's Request for Further Information. However, during the trial, I gave leave to Dyson to amend their case (by consent, subject to the usual order as to costs) so as to introduce into this letter the following sentence (by reference to an attached drawing), the amendment being underlined:

35. Stage 1 refers of course to SU1 etc.

89 By the close of evidence, Hoover relied on three reasons why claim 1 was not infringed:

36. I shall deal with (a) and (b) together as they are basically aspects of the same point.

90 I shall first briefly digress to the topic of 'uniflow' cyclones which were mentioned in para 43, where it may be recalled, I held that awareness of such devices did not form part of the common general knowledge at the priority date. In HTV, it is agreed that SU2 acts as such a device. Prof Allen considered uniflow cyclones in his Report (paras 39-41) in some detail. In these devices, static vanes cause the particle-laden gas to spin within the unit at the point of entry. The spinning gas then rotates down the walls of the device and exits at the end of the unit opposite to its point of entry. In so doing, particles entrained in it experience centrifugal forces tending to make them move toward the wall of the vessel. Prof Allen states (para 39):

37. The scavenge gas flow is drawn off to carry the dust laden air (usually 15-20% of the gas flow) out of the system and on to a second cyclone where particulate matter is collected. This so he says, enhances efficiency by minimising re-entrainment.

91 In HTV, all the pre-cleaned air from SU1 enters SU2 where it is first subjected to such a cyclonic concentrating action. The larger part of this air is discharged or 'bled off' out of the top of SU2 as reasonably clean air, after which it is filtered before being finally exhausted, thus by-passing SU3 altogether. A concentrated dust stream which of course requires to be rid of dust (being some 25-30 % of the air flow from SU1) is nonetheless passed to SU3. SU2 therefore creates two air streams but deposits no dust. All of the concentrated dust stream which enters SU3, will of course have already been pre-cleaned in SU1. Thereafter some of the air from SU3, so Hoover says, is returned to SU2 for re-cycling ('re-circulation'). The majority of the upward air vortex cleaned by SU3 exhausts centrally through SU2 – together of course with the bleed off from SU2. Hoover can tolerate this apparently inefficient release of air still laden with some fine dust from SU2, by the judicious use of special 'depth filters' (Allen Report, para 110). Significantly, Prof Allen said that SU2 and SU3 'interact as part of an overall system': see para 98 below.

92 Hoover assert that the HTV appliance does not infringe. It does not have the 'lower efficiency' cyclone unit SU1 connected to the 'highest efficiency' unit in series because the Patent claim requires each cyclone unit 'in the train' to be of successively higher efficiency. SU1 and SU3 are quite simply not connected in series ie directly, in literal 'one-after-the-other' communication. SU2 intervenes and moreover, affects the overall mode of operation of the appliance. SU1, SU2 and SU3 are indeed in series connection in that order, but say Hoover, that is not what is claimed.

93 Hoover agree however (see above) that SU2 not only deposits no dust but does not even have the capability of so doing. They also admit that SU1 is a cyclone unit of lower efficiency than SU3.

94 I reject Hoover's submission that SU1 and SU3 are not 'in series connection'. First, ignoring for a moment re-circulation from SU3, it is agreed that dirt-laden air flows sequentially through the appliance from SU1 through SU2, on to SU3 and then out of the cleaned air exit in SU2 to the environment. Hoover have chosen to use SU2 to bleed off three-quarters of the pre-cleaned air from SU1, out to the surroundings. This air still contains some fine dirt and this is an inefficiency which is rendered acceptable by the subsequent use of filters. Nonetheless, there is a concentrated dirty air stream which must next be cleaned in SU3, albeit of less volume than that of the bleed off. But the significance of this dirty air cannot properly be measured merely in terms of volume; it must be looked at in my judgment, in terms of the overall purpose of the apparatus. This is in truth the functional sequence which is at the very heart of the vacuum cleaning appliance subject of the Patent. However, I have held that SU2 is not a 'cyclone unit' within the claim; it does not deposit dirt nor is it even capable of so doing. It is therefore not the 'highest efficiency unit' and so far as claim 1 is concerned, even if its presence is in some way beneficial. It does not 'count' so far as infringement is concerned. All dirt-laden air entering SU3 has first been pre-cleaned by SU1 which is upstream of SU3; SU3 alone is the relevant 'highest efficiency unit'. It matters not that such pre-cleaned air has come, slightly concentrated perhaps in dirt, through SU2 any more than it has passed through say, specially designed ducting from SU1. SU1 and SU3 are in series connection as called for in the claim. As noted, the fact that this concentrating action may be either an advantage or a disadvantage, is irrelevant if, as is the case, it does not affect the way the invention works: see col 1, ll 63 et seq. Furthermore, the presence of SU2 is not in my judgment, a 'variant'. In reaching this conclusion, I have taken into account the evidence of both Prof Syred and Prof Allen.

95 Next, I shall consider re-circulation from SU3, noting at the outset that Dyson do not accept that this occurs, save perhaps to a de minimis degree. I shall consider the parties' experiments on this topic below but for present purposes I shall initially assume that some re-circulation from SU3 does indeed take place. Hoover's argument as I understand it, is that the word 'sequentially' in claim 1 requires a progressive unidirectional airflow throughout the cyclone units and that re-circulation vitiates this requirement.

96 In my judgment if it occurs, re-circulation is not relevant to the issue of infringement. First, even if there is re-circulation, SU1 and SU3 are still in series connection, for the reasons previously given. The dirt laden air, the sequence of whose passage is important, is that which has left SU1 after pre-cleaning. The fact that after leaving SU3 the cleaned air may be subjected to yet further cyclonic cleaning (in this case by re-circulation, but equally in theory by further higher efficiency cyclone(s) downstream of SU3) does not avoid infringement. If re-circulation is a variant, it certainly does not affect the way the invention works, and obviously so, having regard to what the skilled addressee would know about cyclones. Furthermore, it is certainly not excluded by the claim having regard to the proper meaning of 'cyclone unit'.

97 I next turn to the alternative way in which Dyson put the infringement argument which became available as a result of the amendment to which I have refered. They say that SU2 and SU3 may properly be regarded as a single 'cyclone unit' within the meaning of the claims. All the output of SU1 undoubtedly goes into such unit, it is undoubtedly in series connection with it even on Hoover's construction, from it dust is deposited and clean air is exhausted to the environment. I confess to having at first viewed this alternative argument with some suspicion as it so easily avoids the complex issues of bleed off and re-circulation upon which the parties devoted so much time in preparation for trial. On this view, the internal 'plumbing' in this part of HTV can in effect, be ignored. However having considered the evidence, I believe that Dyson's alternative argument is sound. There was some skirmishing between the experts as to whether this sort of 'block unit' approach was in theory (that is, from essentially the academic point of view) legitimate but I do not consider it profitable to approach the matter this way.

98 I have of course already held that the phrase 'cyclone unit' can comprise a number of cyclones. Furthermore, Prof Syred ( First Report para 164) was of the view that the combined unit SU2/SU3 " has a higher efficiency in the capability of depositing fine dust than SU1." There was no cross-examination on this. More importantly however was the evidence that on a technical (and not just a notional) basis, SU2 and SU3 acted together as a single unit. Prof Allen for example, under cross-examination, stated that:

99 In case I should be wrong on the question of re-circulation, I must turn briefly to the parties' experiments on re-circulation– though I noticed that relatively little time was taken at trial over them. Dyson's Experiment 1 involved the insertion of a tube into the exit of SU3 in a test rig so as to try to ensure that there was no re-circulation. Prof Allen agreed that this would have the desired effect of eliminating re-circulation. He said

38. I have considered all the evidence on Dyson's Experiment 1 together with the evidence relating to the deposition patterns on the top plate used in the Experiment 1 test rig. I have also taken into account Prof Syred's calculations which are exhibited to his report as NS-8 (and the corrections thereto which became Document X-18a) and his evidence on X 9 regarding re-circulation. I have also reviewed Hoover's experiment in reply. The conclusion I have reached is that whichever way one looks at the matter, if there is any re-circulation it is likely at most to be de minimis.

100 Hoover vehemently assert that the part of SU3 which separates the finest dust from the air stream is not frusto-conical but is 'trumpet shaped' and thus outwith the claims. HTV they argue, does not infringe for this reason alone. No evidence was tendered by Hoover, technical or otherwise, as to why this shape was chosen though Prof. Allen has speculated that it ought to be because Hoover thought it would give better separation.

101 The expression 'trumpet shaped' is itself not an exact descriptive phrase. Different sorts of trumpets have different profiles. Furthermore, the horn, which sounds an octave below the trumpet, is also visually in the same league and for my part I should have described the separating zone of HTV (see for example Annex III) as being more like a cut off English hunting (or beagle) horn. Certainly, both are tapered. The apparently trivial difference between a geometrically perfect frusto-conical profile and that of the relevant portion of the HTV was dramatically highlighted at trial by the drawing put forward by Dyson in chief as Document X-9. This was a blown- up photocopy of the relevant portion of a sectional drawing of the relevant part of the HTV appliance. Upon this, two geometrically correct frusto-conical profiles had been superimposed in red, showing a slightly different notional angle corresponding to the 'top' and 'bottom' of Hoover's 'trumpet'. I have held that properly construed, the phrase 'frusto-conical' does not demand such geometric exactitude since to do so would be to indulge in the sort of strict literal interpretation which is forbidden by the Protocol. For this reason alone, I would be hesitant to hold that the separating zone in SU3 is not in fact 'frusto-conical' within the proper meaning of the claim. And I feel sure that the skilled reader would so regard it.

102 Assuming however that this is indeed a 'variant', I shall examine the parties' positions and the experts' evidence on this mater. First, does the variant affect the way the invention works? There was a good deal of evidence on this point. Prof Syred's position was that the trumpet shape made no difference at all to the way the separator worked. It worked in just the way that I have held that a skilled worker would have understood a frusto-conical cyclone separator to work at the priority date. Prof Allen was unwilling to give a definitive view on the matter. As previously noted, he was however of the view that the 'trumpet' shape might even enhance the normal separative effect of the conventional frusto-conical shape. In a supplementary Report, Prof Syred commented on this evidence of Prof Allen thus:

39. Professor Syred was not cross-examined on this evidence. The diagram to which I have already referred, X-9, was put to Prof Syred in chief at 1/130. His evidence was to the effect that the performance of the trumpet as a particle separator would be somewhere between the two strictly geometric frusto-conical profiles which had been superimposed in the diagram in red. He was not I think, cross-examined on this. In the end, Prof Allen agreed with Mr Kitchin QC on the question of expected performance

103 In my judgment, on this evidence, I do not believe that the 'trumpet' shape of the separating zone of HTV affects the way the invention works. Furthermore, I would expect that that would have been obvious to the skilled addressee with his basic knowledge of cyclone separation at the priority date. Finally, as I have held earlier, there is in my judgment no particular comprehensible reason why strict compliance with the geometric meaning of 'frusto-conical' would have been understood by the addressee to have been required.

104 In my judgment therefore, Hoover's HTV infringes claim 1 of the Patent

105 Hoover have admitted that SU3 is 'within' SU1. In the light of my construction of the term 'within', it seems to me clear that the combined 'cyclone unit' comprising SU2 and SU3 must also be 'within' SU1 in the HTV. In my judgment, Hoover's HTV therefore also infringes claim 2 of the Patent.

106 Hoover have admitted all the other integers of the claims in suit. It follows that Dyson have succeeded in the action for infringement of the Patent. I now turn to Hoover's counterclaim.

VALIDITY

107 By section 72(1)(a) of the Act, a patent may be revoked if the invention is not a 'patentable invention'. What is a patentable invention is established by section 1(1), sub-sections (a) and (b) of which are relevant to this counterclaim and provide:

46. These conditions are commonly spoken of however in the negative, in terms of 'lack of novelty' and 'obviousness'. Both are in issue in this case.

108 In fact, three grounds were relied upon by Hoover to attack the validity of the Patent: lack of novelty, obviousness and insufficiency. I shall deal with them in that order. On the issue of validity, the onus is of course upon Hoover to prove its case.

109 In relation to lack of novelty and obviousness, the attack is based entirely on certain pleaded prior publications; there is no issue as to prior user. These prior publications are as follows:

and in relation only to claim 1 of the Patent,

47. Citations (c)-(e) relate to essentially the same item of equipment being an apparatus for dust collecting during drilling operations in underground mining. In addition to these citations, Hoover relied on common general knowledge in its attack on obviousness.

110 By section 2(1) of the Act, an invention shall be taken to be new if it does not form part of the state of the art. The state of the art is defined by section 2(2) to

111 There was common ground as to the principles of law which should apply to an attack on the ground of lack of novelty. First, from the judgment in General Tire v Firestone Tyre [1972] RPC 457, there must be clear and unambiguous directions in the prior publication to do or make something which falls within a claim under attack. In order to do this, it is of course necessary first to construe the prior proposal in question. Sachs LJ, delivering the judgment of the court said this (at pp. 485-486):

Earlier, in Hills v Evans (1860) 31 LJ Ch 457 at 463, Lord Westbury said:

48. The latter statement was qualified by Lord Reid in van der Lely v Bamford [1965] RPC 61 at 71, to mean "experiments with a view to discovering something not disclosed".

112 In relation to lack of novelty, as will be seen, on one citation Hoover have relied on an experiment on the basis of 'inevitable result', as it is sometimes called. In that connection I would say this: The interpretation of a prior art document is deemed to take place as of the date of its publication. Its interpretation at that date is as always, for the appropriately skilled addressee. That interpretation must not however be supplemented by what has occurred between the date of publication of the document in question and the priority date of the patent in suit: see per Sir Lionel Heald QC in 3M v Bondina [1973] RPC 491 at 521-523 and the cases cited therein.

113 To these authorities I shall add the well-known 'WORKMATE' case on which Mr. Kitchin relies: Hickman v Andrews [1983] RPC 147. The device subject of the patent in issue in that case was a 'workbench' which combined a workbench, a saw horse and a vice with elongated jaws. The defendants counterclaimed for revocation of the patent inter alia on the ground of lack of novelty in the light of the prior use of a device which was a small bookbinder's finishing press known as a Russell Press. Every claim began with the words 'A workbench' and lack of novelty depended essentially on whether the Russell Press was indeed a 'workbench'. Both the trial judge (Graham J) and the Court of Appeal held that this question was to be answered in the context of how the word was used in the specification. As Graham J put it: " I think one must be realistic about these things" (p168). Having determined the proper meaning of the word 'workbench', the Court of Appeal upheld Graham J's finding that the Russell Press was not a 'workbench', as it was 'far too small to be so' (p. 186). On the same point, Graham J held that:

114 The van Berkel citation, US Patent no. 2,171,248, was not relied on under this ground of objection.

115 US Patent no. 2,768,707 (Campbell)

49. This patent has a filing date of 1954 and was published in 1956. There is no evidence that its teaching was ever put into effect. Before considering this citation, I must make this observation: that it is in my view absolutely vital when reading this patent to do so as the notional skilled man would have done in 1956, that is in particular, without any knowledge whatever of the Patent. Why I say this will soon become apparent.

116 Cambpell is entitled 'Separator for use with Vacuum Cleaning' and is devoted to a single apparatus, the drawing for which I have attached hereto as Annex IV. Campbell's machine is an industrial unit (both experts were agreed on this) which has a dual purpose. First it can separate 'dust, finely divided particles and the like, such as in connection with vacuum cleaning apparatus'. Secondly, it is destined for use in the spray-drying field to agglomerate 'dried dairy and food products in particle form, and other materials carried by a fluid medium, such as those sprayed into a drying chamber.' ie as a product separator. The objects of the invention are said to be achieved 'at extremely high operating efficiency' and at ' an extremely low pressure drop'. The patent continues (col 1, ll 37-45):

50. This is brought about by a fixed 'primary whirl-promoting tuyere' connected to a conventional conical cyclone separator (14). The overall movement of air through the apparatus is effected by suction pumps, fans and the like. The absence of moving parts, coupled with ease of dismantling for cleaning or repair are spoken of as being further advantages. The path of the laden air through the apparatus is indicated by arrows, particularly those 'within the tuyere and the cone' (Col 3, ll 20-24).

117 The description of the machine is fairly straightforward. Air or vapour laden with the particles whose separation is required passes into a 'separation' or 'receiving and separation' chamber (2) from

51. This 'laden fluid' is conveyed into the separation chamber by a hose (7).

52. Pausing there, Campbell indicates alternative modes of delivery of the 'laden air' into the separation chamber: radial or tangential. The tangential mode is illustrated and shows a short, curved pipe protruding about a quarter of the way into the separating chamber. But in my judgment, the reader could expect high efficiency separation by either proposal. Two arrows showing the path of laden air emerge from this curved pipe, one going upwards, the other downwards. Thereafter under the influence of the low pressure drop, the 'laden fluid' from the inlet conduit 7

53. The tuyere consists of tangential blades in a cylindrical body which impart a strong (even 'violent', according to col 4, line 3) whirling motion to the laden air. I believe the reader would notice the contrast between this strong/violent motion and the initial rotary motion of the same laden fluid as it enters the separation chamber if the option of tangential entry is taken up. The swirling air now enters what I shall for convenience call a conventional separating cyclone of the kind which the skilled man would know about: see para 41. This is clearly intended to be the 'guts' of the device. Here, some dust will be flung 'centrifugally' to the sloping wall of the cone to be deposited 'directly downwardly along the cone inner side to the discharge end of the cone'(col 3, ll 35-40). Significantly, the lower part of the cone is where agglomeration of material takes place (col 3, ll40-46) again leading to further deposit of the same material in the discharge end of the cone. Here it is temporarily stored, the deposited material being intermittently discharged via a spring-loaded flop -valve (17). In the diagram, a mound of discharged material is shown directly below and co-axial with the discharge port of the flop valve (only). The material accumulates in a pan (5) (which is removable by using the handle provided (6)), 'for dumping into some convenient container.' One can incidentally, gain some idea of the scale or size of the device from this handle. The two patent claims refer to the action of the cyclone as providing 'means for progressively agglomerating particles in the whirling stream leaving the tuyere'.

118 Just as the skilled reader would expect with a reverse flow cyclone, the cleaned fluid whirls upward through a vortex finder (18) into a discharge chamber (3) and is finally exhausted via filters 23,25 and 26.

119 In my judgment, the reader of this document would appreciate that it is via the tuyere and within the central conical cyclone alone that the benefit of efficient separation and/or agglomeration takes place. Prof Syred agrees with this: Report, para 204. Hoover argued however that because the 'separation chamber' was so called, that indicated that at least some separation must be taking place therein, that is, around but outside the central conical cyclone. I reject this argument. The separation chamber (elsewhere called the 'receiving and separation chamber') is indeed where separation occurs- but within the central conical cyclone which is itself wholly within the separation chamber. The descriptive distinction is between the separation chamber (2) on the one hand and the discharge chamber (3) on the other: Col 2, ll 15-16. Note also that there is also no mention of 'dirt' in the Campbell patent. The skilled reader would I believe, understand this device to be an industrial proposition for the collection of all the particulate material in the single bin, that material being the wanted product. The agglomerating and the vacuum cleaning functions in Campbell are proposed on equal footing and are there to treat laden fluid passing through separator. As Prof Syred said "The obvious intention was to recover as much of the spray dried milk or egg material as possible" [Supplementary Report, para 15 (e). No cross-examination]. There is no suggestion of using Campbell to clean dirty surfaces, for example.

120 In order for Campbell's proposal to be an effective citation under lack of novelty, Hoover have of course first to establish that the walls of the cylindrical separation chamber act as a cyclone. Then they have to establish the lower efficiency of this unit with respect to the reverse flow cyclone - which is undoubtedly described. In my judgment, on the face of the document, Hoover have failed to do this. The possibility of cyclonic separation adjacent the walls of the separation chamber is neither described, illustrated nor, reading the patent as a whole, even contemplated. First, the swirl, a necessary pre-condition to cyclonic action, would only arise so it seems, when the tangential rather than the radial option on the inlet is exercised. Yet the machine one would be entitled to assume, works in just the same way using either option. Setting up a swirl within this outer chamber so as to effect separation would I believe, be if anything counter-indicated by reason of the presence of the unprotected mound of the (probably desirable) material under the flop-valve. In any event, Campbell's is a system which aims for 'high operating efficiency' of the central cyclone. This is literally the focus of the proposition. The presence of true swirl in the separating chamber (which is not I believe, the same as the 'initial rotary motion' described for the tangential option) would be likely to disturb this material, whether falling on the actuation of the flop- valve or in repose, leading to re-circulation into the cyclone via the tuyere, thus leading to inefficiency. Furthermore, if separation were to have already occurred to any significant extent in the separating chamber, this would also be likely to diminish the efficiency and purpose of the central cyclone. As a minor point, this view is further supported by the direction of a downward arrow in the diagram adjacent the bent inlet tube.

121 What then is the purpose of the separating chamber? There is some evidence on the point and in this matter, I prefer the evidence of Prof Syred to that of Prof Allen. It is more realistic and accords in my view, with a fair reading of Campbell. Prof Syred in his Report sees Campbell's separation chamber simply as a plenum chamber. As he put it (Supplementary Report, para 13 ):

122 Hoover did not however let the matter rest there and decided to attempt to prove their case by an experiment, in order to show that 'Campbell's outer chamber inevitably act[ed] as a cyclone to some extent' : Hoover's Opening Skeleton, para 48 (cf General Tire, para 109 above). As a result, there took place a late round of experiments by both parties, with leave of the court, based on the views of each as to what Campbell must have intended and how it functioned. The parties made what they believed were working models of Campbell and tested them. They also made video recordings of their experiments (which I have seen). I must state however that in their experiments, Hoover chose to ignore Campbell's first option as I have called it, that of introducing the laden air radially into the separating chamber (col 2, ll 26). In what follows therefore, there is at the outset, an element of ex post facto selection on Hoover's part as to which option was likely best to assist their case. This is not I would suggest, have been the likely approach of the skilled man in 1956, the date of publication of Campbell.

123 The purpose of Hoover's model/video/experiment was recorded in their Notice dated 5 June 2000, thus:

54. What Hoover did was to make a model of Campbell of a such a size that it could be fitted to a standard Dyson model DC 01 vacuum cleaner, the latter having been modified so as to accommodate the Campbell addendum. Hoover also modified Campbell's machine in other ways eg by sealing the flop valve with tape. They then ran two tests on this hybrid machine using a three part test dust having components above and below 50µ and 'ABS moulding chips'. There was no evidence that either of these were available in 1956. In the second test they used strips of coloured paper alone, to show flow patterns. They made a video recording of the result. The outcome of the tests was said to show :

55. From the video recordings, I could see (b) and (c), but not (a) with any certainty. I could however see particles in the outer chamber swirling quite violently in apparent cyclonic motion and am prepared to accept that some at least of these were being deposited in the separation chamber as a result of purely cyclonic separation.

124 I should say that Prof Allen twice placed on record the fact that he did not design the cyclone used for the Hoover experiment: 4/543 and 547.

125 Dyson were highly critical of the scale of the Hoover cyclone (in particular of the flop- valve and of the delivery arrangements for the artificial 'dirty' air to enter the machine contending (Prof Syed Second Report, paras 24-29) that it

56. In response, they made a model of Campbell themselves 'at the bottom end of what the document contemplates', and performed an experiment using it. Their notice, dated 7 July 2000, seeks to show the following (I have abbreviated the wording):

126 Details of the Dyson 're-build' are set out in Annexe I to Dyson's Notice. It is at once apparent that Dyson have built a device about half the height of a man. They have also chosen to deliver the particle-laden air not via a 'tangential' inlet of the sort depicted in the Campbell drawing but 'radially of the chamber'. This it will be recalled, is the alternative (col 2, lines 25-27) to the arrangement which Campbell has drawn. From the video recordings which were made, I could see the bits of paper and moulding chips falling apparently by gravity, to the bottom of the separating chamber. The chips fell directly down and the paper spiralled down, no doubt meeting some air resistance. Some R 10 dust also appeared to be deposited in the base of the outer chamber but far more was deposited on the outer walls of the separating chamber and within the conical separator. But as with Hoover's experiment, I could not observe (a). What I could see however, was a slowly rising serpentine coil of particulate material which was swirling just inside the walls of the separating chamber, but not substantially in contact with them. Hoover allege that even Dyson's model of Campbell anticipates claim 1 of the Patent.

127 Needless to say, these experiments gave rise to a lively divergence of opinion between the experts as to the cause of what was being observed. In turn, this gave rise to some of the most complex technical aspects of the case.

128 In repeating any prior art proposal by experiment with a view to proving lack of novelty, the party seeking to do so must in my judgment adhere faithfully to the teaching of the source document. Moreover, the interpretation of the cited document for the purpose of such experiment is deemed to be assessed as of the date of its publication - in this case, 1956. The interpretation is also deemed to be the job of the skilled worker, whose knowledge must not be supplemented by knowledge which he might have obtained between that date and the priority date of the Patent, here, 1980. As Sir Lionel Heald QC said in 3M v Bondina [1973] RPC 491 at 521:

57. The authorities he referred to included the passages from General Tire to which I have already referred. I would add that Sir Lionel Heald's reference to 'the specification of the patent being attacked', must apply equally to a physical embodiment of the invention claimed therein.

129 Before considering the evidence on these experiments in detail, I must record that I regard the manner in which Hoover chose to carry out its experiment as not being in accordance with the requirement as to interpretation noted in the previous paragraph. Hoover did not in my judgment attempt objectively to interpret Campbell as of 1956 without knowledge of Dyson but rather, was more concerned at all costs to find the Dyson invention somewhere within it. It is clear that Hoover did not faithfully build the device described by Campbell and published in 1956. They built a scaled-down and amended version of Campbell specifically to co-operate with an emasculated Dyson DC 01 vacuum cleaner of the late 1990's. I have no reason to believe that the Dyson DC 01 machine does not fall within the claims of the Patent under attack. Moreover, Hoover did not make use of Campbell's working materials viz 'dust' and finely divided, dried dairy and other food products. Dyson make other criticisms of the faithfulness of the Hoover 're-build' and the experiment thereon (such as inlet air velocities and flow rate) which may also be justified (see below). If reliance is to be placed on experiments of this character, the directions in the prior art teaching must be adhered to and as if by a skilled worker of 1956. Hoover did not do this and for that reason alone, its experiment with Campbell is in my judgment, fatally flawed. In case I should be wrong about that conclusion however, I shall now consider these experiments in a little more detail and record my factual conclusions on them.

130 My review of the considerable evidence given on this aspect of the case leads me to conclude that on the balance of probabilities, Hoover have not proved that realisation of the Campbell disclosure will "inevitably result in something being made or done which, if the patentee's patent were valid, would constitute an infringement of the patentee's claim." The following are the principal factual reasons for my coming to this conclusion.

131 First, there is the question of the scale of the alleged re-constructions of Campbell. Even allowing for some animal hyperbole in Mr Kitchin's jibe that "They saw an elephant and drew a mouse", the scale of the Hoover model is in my judgment does not at all reflect what Campbell had in mind. The Campbell proposal is in my view (and that of both experts), a device for industrial application and proper account has not been taken of this fact in the scale adopted by Hoover-though it has by to some extent by Dyson. I reject Hoover's contention that 'Dyson have made a Jumbo version of Campbell'. I have already drawn attention for example to the size of the handle (6) in the Campbell drawing. In my view, Dyson's model is nearer the mark in scale to Campbell's industrial device and in this connection, I agree with the evidence of Prof Syred in his Supplementary Report, paras 6 and 8 and his rough characterisation of Hoover's model as a 'micro' model and Dyson's as 'mini'. This change in scale will according to the evidence, have at least some effect upon what goes on within the device-though for present purposes, I need not I think, attempt to evaluate the extent of it. Then there is the placing of the Hoover device on the emasculated Dyson DC 01. I have already noted that in my view, this is simply wrong; the relevant date for the 're-build' should have been 1956, not 1999. This is bound to have a 'follow-on' effect in reducing the flow rate. Prof Syred was adamant on this: 3/139-141. In addition, both parties sealed the flop valve which is not what Campbell proposes. In addition, the flop valve in Hoover's model, was in Prof Syred's view absurdly small: 3/143. Moreover, the moulding chips which Hoover used could not all have gone through the tuyere slots anyway, because of their size and in the Hoover device, the inlet was not I think, quite as drawn in the Campbell figure.

132 Prof Syred was also cross-examined at length as to how a visible layer of particulate matter ("a light coating") came to settle on the walls of the separation chamber of Dyson's model of Campbell: 3/300-308. Hoover alleged that this came about by cyclonic action. However, Professor Syred was of the view that electrostatic forces had been primarily responsible for this: 3/308. If there was any cyclonic effect, it was in his view "negligible". Prof Allen was of course more inclined to attribute this to a cyclonic phenomenon operating in the separation chamber. However this light coating was not deposited where the dust or agglomerate should have been deposited as a result of cyclonic action, that is at the bottom of the collecting pan. Then there is the issue as to whether the paper and moulding material dropped into the bottom of Dyson's version of the Campbell separation chamber by gravity ("an inertial separator or drop-out pot" : Syred Supplementary Report, para 35) or got there as a result of centrifugal separation. Professor Syred remained of the view under cross-examination that this was indeed a gravity separator. In fact Hoover agreed that the moulding chips did fall by gravity but they said, this was because they had been chosen so as to be too heavy to begin with. The evidence of both experts on this entire latter point is too speculative for me to come to a sure conclusion as to who is correct.

133 I have sought to highlight a number of the factual issues which were in debate at the trial with regard to what Campbell's device showed (or did not show) and how it operated. I have not however sought to resolve what were often differences of opinion on complex questions of cyclone technology and the causes of transient fluid flow phenomena, since I believe the task to be unnecessary. Even at this stage, in my judgment, it cannot be said either that Hoover have faithfully re-built Campbell according to the criteria discussed above or that the results of Hoover's experiment show on the balance of the evidence that what was going on in the separation chamber of their model was due to cyclonic action.

134 There is however another more fundamental reason for rejecting lack of novelty on the basis of Hoover's experiment. According to the evidence, what is the most significant practical parameter in this unit is the inlet velocity of the laden air as it enters the separation chamber. Both experts were agreed that this is of great importance: Prof Syred, Supplementary Report, para 10: "The important parameter is the inlet velocity to the outer chamber." Prof Allen in cross-examination 4/465 (see below for a fuller quotation). There was a significant difference between the inlet velocities as between the two devices even allowing for debate over exact figures: see Prof Syred, Supplementary Report, paras 33-34 and cross-examination 3/316 et seq. That however is not quite the point. The point is that changing the inlet velocity of the particle-laden air could significantly alter the mode of action of Campbell's separation chamber, both as regards what happens within the reverse cyclone and within the cylindrical chamber alone. At 3/300, Prof. Syred said:

At 4/564, Prof Allen said:

135 It seems to me that there are evidently a number of ways of operating Campbell which will depend upon choice of the inlet velocity. Some may fall within a claim of the Patent, others may not. Following General Tire, that evidence alone is enough to dispose of the argument of 'inevitable result' and thus lack of novelty. Further, a fair reading of Campbell leads Professor Allen to conclude that it was intended for a particular application wherein separation in the outer part of the separation chamber was actually to be avoided because of break-up of the working particles. That is, that it is to be operated in a manner which would not fall within any of the claims of the Patent. It must be borne in mind that the criterion in lack of novelty is that of inevitability and not 'might'. Furthermore, the words of Jenkins LJ in Fomento v Mentmore [1956] RPC 87 are particularly germane to this aspect of the case. At page 101 of that report, dealing with lack of novelty, he said:

58. There are therefore in my judgment, no 'clear and unmistakable directions' in the Campbell patent, no planting of the flag 'at the precise destination' and in addition, no 'inevitable result' in the light of Hoover's (or Dyson's) experiment with a Campbell 're-build'. The objection to the Patent on the basis of lack of novelty in the light of Campbell, accordingly fails.

['Johnston/Donaldson']

136 There are actually three separate citations here, each of them ) under this head against claim 1 only. The inventor of the US patent was one Johnston who assigned his invention to the Donaldson Co Inc of Minneapolis ('Donaldson'). His patent relates to a 'Discharge Valve Assembly for Multiple-Stage Dust Collector' and was published shortly before the priority date of the Patent. The other two citations also emanate from Donaldson and concern dust control apparatus provided in a similar but earlier machine, the Donaldson Roof Drill Dust Collector. This machine seems to have been built and put into commercial operation. These earlier citations are the Donaldson Bulletin (1973) and the Donaldson Manual (1975). Under obviousness (see para 173 below) the same trio of documents is relied upon to attack all the claims whose validity Hoover seeks to impugn.

137 In general terms, the apparatus described in all three publications is the dust-collecting part of a roof bolter used in underground mining operations. The machine into which it is mounted is illustrated on page 6 of the Manual and appears to be essentially the same machine as that illustrated as Fig 1 of the patent. It is a machine of substantial size, mounted on wheels and is used for drilling holes in the roof of mineshafts. What immediately struck me about this machine is that it is certainly not a vacuum cleaning appliance. On the contrary, it is a mobile drill which has inter alia a dust extractor/air purifier incorporated into it, that is, as part of it. At the outset, Dyson submitted, accurately in my view, that this machine is in truth an overall generator of dust and to characterise it as a "vacuum cleaning appliance" would simply be an abuse of language. Its dust control facility is, they say, primarily to protect the lungs of mining personnel. Nonetheless, the Bulletin at least is mainly devoted to a description of the air purifying capability of this machine alone and so the matter must be examined more closely.

138 The patent explains (Col 1, ll 42-55) that in modern mining, the shaft roof is no longer held up by props but by beams which are bolted to the roof of the shaft. These beams are secured to the roof by bolts which are drilled into higher and firmer strata above the roof. To do this, holes have to be drilled and Johnston provides a six-wheeled truck, bearing the relevant equipment for doing this, including the dust control unit.

139 According to the Bulletin, dust control is accomplished in three stages by means of a unit which is described as being both 'rugged' and 'compactdesigned to fit existing roof-drilling equipment currently in mine operation'. The first stage is a 'tangential cyclonic separator' which is illustrated. It is said to remove over 95% of the dust entering the dust collector and is cylindrical in cross-section.

59. Thereafter, a multi-cyclone unit is provided, which is also illustrated being used to strip the drilling air of coal dust. Each unit of this second stage is a conventional reverse flow, tapered cyclone. 'Once these large particles have been removed', this second stage separates more than 84% of what gets past the first stage. Finally, there is a barrier type filter element. The bulletin states:

60. Needless to say, Hoover fasten onto this last item of information in arguing their obviousness case.

140 The Manual as its name suggests, is a user's guide for the Donaldson Dust Collector and adds detail to what is contained in the Bulletin. There is no cross-reference to the Bulletin, however. The single arm roof bolter is now illustrated (Fig 1), the final filter is said to collect 'extremely fine' dust (less than 15µ in size), and a recommendation is given for the value of the suction.

141 In so far as dust collection is concerned, the cited Johnston patent differs from the proposals in the Bulletin and the Manual in relation principally to the construction and geometry of the second-stage, multi-cyclones. Significantly, it refers in this connection to a slightly earlier patent of Johnston, US Patent no. 4,144,043 ('043), which is incorporated into the Johnston patent by specific reference (Col 6, line 23) and which is directly concerned with this aspect of improved dust control. In relation to the prior art, '043 itself twice refers to the cited 'Operator Manual' (col 1, ll 20-25 and col 6, ll 67-68). The parties referred to '043 and cross-examined on it.

142 '043 highlights a shortcoming in the dust collecting aspects of the device described in the Manual viz its poor 'down time' when the final filters need to be changed and a number of practical considerations are reviewed as to how to minimise roof drill 'down time' (see col 1, ll 33-55) . One of these is to 'simply increase the size of the dust collection chambers' but

61. The advantages of a 'reduced-size package' are spelt out in col 6 lines 63 onwards. Prof Allen was asked this in cross-examination concerning col 1,ll 43- 55 (4/500):

62. The solution which Johnston proposes in ' 043 is to replace the earlier tapered multi-cyclones of the second stage with a so-called 'side arm' cyclone, which is distinctly more compact. And this is what is used in the Johnston patent which has been cited.

143 There was disagreement between the experts as to the viability, efficiency and mode of operation of Johnston's side arm separators, since such devices have certain inherent problems. For example, Prof Syred was downright critical of the side arm proposal in its entirety . Furthermore, Dyson argued that the side arm cyclone was not a true cyclone having a frusto-conical part at all and drew upon the evidence of Prof Allen in cross-examination for this purpose. Prof Allen there seemed to subscribe to the view that if separation was taking place in the side arm cyclone at all, it would be likely to occur in the upper cylindrical part alone. The matter was however even to him, well within the realms of abstruse cyclone theory.

144 Whatever be the true scientific position, I find that to the skilled reader of Johnston at any rate, all this evidence would never have occurred to him. Just as he would be likely to consider that a 'trumpet shaped' cyclone would operate as a particle separator in essentially the same way as a frusto -conical cyclone (see above), so too I believe that seeing the diagrams in the Johnston patents, he would think that these would be likely to behave in the same way.

145 It is worth recording at this juncture that Johnson's '043 patent has this to say about the overall purpose of the dust collection:

63. The cited Johnston patent has for its prime purpose, the provision of a particular type of discharge valve in the dust- collector described in the earlier patent. But in this case, Hoover rely only on the teaching of the citation in relation to the dust collecting apparatus. In my judgment, the overall purpose of all the Johnston/ Donaldson proposals whether read apart or together, is to provide an effective means to rid the air in mines of substantial quantities of noxious mineral dust for the benefit of those who work therein- 'to protect the mine personnel' as the Bulletin puts it.

146 I do not need to go further into the complexities of the Johnston/Donaldson proposal since, by the end of the trial only two issues remained on the question of lack of novelty. First, whether the device can be called a 'vacuum cleaning appliance' at all (Claim 1, integer (a)). Secondly, whether the second stage separator is of higher efficiency in its capability of depositing fine dust (Claim 1, integer (b)). I shall deal very shortly with the second issue first.

147 The Bulletin (p.1) refers to the first two separators as 'pre-cleaner sections'. The first stage, the cylindrical separator, removes 95% of the dust entering the dust collector. 'This rugged pre-cleaner collects the large particles of dust and deposits them in a drawer under the pre-cleaner' (p. 2). As one would expect, the drawer is capacious (Photograph, p.1). The Bulletin says of the second stage: 'Once these large particles have been removed, the remaining dust. [passes] into the second stage' (p. 2). The information given in the Brochure is to the same effect: 'main dust' and 'small dust' are removed in that order.

148 Of this Prof Allen says in his Report ( para 146):

64. Initially, Prof Syred was not of this view but after some sustained cross-examination on the point by Mr Floyd, he agreed that the second stage in all these devices was likely to be more efficient in depositing fine particles than the first: 2/238-242. Prof Allen remained firm in his view, stated above, even in relation the 'side arm' variant disclosed in the Johnston patent: 3/363. In the circumstances, in my judgment, integer (b) of the Patent is present in all three of these disclosures. I further hold that since this is the most natural reading of the documents for one less skilled than either expert, it would be the likely understanding of a skilled worker reading these documents at the priority date.

149 There remains the question of whether the machine described and illustrated in any of the Johnston/Donaldson citations can properly be termed a 'vacuum cleaning appliance' as called for in integer (a) of claim 1 of the Patent. In my judgment, to call Johnston's dust control apparatus a 'vacuum cleaning appliance' would indeed be an abuse of language, as Dyson argue. Not only is it a misnomer on its face but to the man skilled in the art (and to a layman too, for that matter) its use to describe the dust control apparatusand still worse, the roof bolter in which it is an integral part, would be absurd. Certainly, the machine works by vacuum. But when in operation, it purifies dust- laden air which it aspires; it releases such air, freed from drilling dust by cyclonic separation into the environment whence it came. That is the raison d'être of the device. The general purpose of the proposal is in fact set out in Johnston's patent (Col.1, ll 52-55):

65. Nowhere in this document is the word 'cleaning' mentioned.

150 in my judgment, the Johnston/Donaldson citation does not anticipate claim 1 of the Patent

151 Obviousness or 'inventive step' as the Act calls it (see above), is dealt with in section 3 of the Act:

66. The burden of proof is upon the party seeking to impugn a patent on this ground and this is normally taken up by adducing appropriate expert evidence. In this connection, the party attacking a patent will seek to marshal and prove evidence as to the state of the art in the light of which a particular step is alleged to be obvious. An unusual feature of this case, already noted, is that Hoover have adduced no evidence at all as to the common general knowledge of the art, even though they were in an advantageous position (so one would have thought), to have done so. The only relevant evidence of common general knowledge before me is that of Mr Dyson, being his own experience of contemporaneous events and the inferences which the court can properly draw from it. But that is secondary evidence, which as has been said in a number of authorities, is merely an aid in assessing the primary evidence. I must state at the outset therefore that this lacuna has not assisted the court in assessing the likely status and impact of several prior art citations relied on by Hoover on the mind of the skilled man at the priority date. Hoover were criticised by Dyson in this respect and in my judgment, that criticism is justified. Thus is Hoover's part of the case.

152 As with lack of novelty, the parties were on common ground with regard to the proper approach to obviousness, though each party stressed different aspects of it by reference to relevant authority. I was for example reminded by Dyson of the cautionary remarks of Fletcher Moulton LJ in British Westinghouse v Braulik (1910) 27 RPC 209 at 230:

67. I was also reminded of Lord Diplock's remarks in Technograph (supra, at p 362) where he said:

68. Hoover drew my attention to the impropriety of attaching weight to the evidence of what the inventor did since (for example) he may or may not be the apotheosis of the hypothetical skilled man. As Laddie J said in Hoechst Celanese v BP [1997] RPC 547 at 565:

153 Further on in the Hoechst Celanese case, Dyson drew my attention to a proposition upon which they strongly rely on the question of obviousness, that is, that for obviousness to be established, there must be a motive for taking the allegedly obvious step. Support for this proposition is to be found both from the words of Laddie J on p 272 and from the judgment of Slade LJ in Hallen v Brabantia [1991] RPC 195 at 212 which Laddie J quotes:

69. Laddie J has integrated the above requirement of 'motive' into the well-established approach of the European Patent Office to inventive step, namely, that of enquiring as to whether a particular solution to a problem is or is not an obvious one to take. He cites various authorities and concludes thus:

70. In my judgment, if at the relevant time, the skilled man perceived no problem which required to be solved, the task of the protagonist wishing to establish that the taking of a particular step (or a fortiori, steps) which would bring an item of prior art within the scope of the claim under attack, becomes considerably more difficult. However, as the next authority makes clear, one must assume that the skilled man will approach the prior art citation with interest; there is no question for instance of spontaneous rejection of matter forming part of the state of the art, as at one stage, Dyson came close to suggesting.

154 It was common ground that the issue of obviousness should in general be approached in the fourfold structured manner proposed by Oliver LJ in Windsurfing Int'l v Tabur Marine [1985] RPC 59 at 73:

71. A little earlier the judge observed (p72):

155 The first two stages of the Windsurfing enquiry generate answers which are of equal relevance to all the cited prior art and I shall deal with them now. In paragraph 28 above, I have set out what I believe to be the 'inventive concept' of the Patent. The second Windsurfing enquiry, identifying the common general knowledge which is to be attributed to the 'normally skilled but unimaginative' addressee, has also been considered at an earlier stage in this judgment. However, the assumption of the 'mantle' of this hypothetical addressee (here, a team) in the assessment of obviousness, throws up some issues in this case which are in my judgment unusual and require particular consideration. I shall therefore next consider some issues which have a bearing upon all the citations and which are generally applicable to the objection of obviousness in this case.

A Prejudice in favour of bags

156 In paragraphs 30-47, I have recorded my findings with regard to the hypothetical skilled addressee and the common general knowledge to be attributed to the addressee at the relevant time. I wish however to add this. Common general knowledge has both positive and negative aspects. I have so far considered under this topic, as is customary, only positive aspects of the knowledge with which the skilled addressee is to be imbued. In my view in certain cases (and I believe this to be one of them), negative aspects of knowledge must in approximation to reality, play their part. At the priority date of the Patent, I believe that such was the 'mindset' within the vacuum cleaner industry, no notional, right- thinking addressee would ever have considered the viability of purifying dirt-laden air from a vacuum cleaning operation, other than by means of using a bag or bag and final filter. For present purposes, the addressee is nonetheless deemed to have been presented with (in effect) three items of prior art wherein it is proposed to clean dirt-laden air by means not of bags but by cyclonic action alone. He is also assumed to take some interest in them however inimical the proposals may be to his likely way of thinking at the time. In terms of its impact on the issue of obviousness, I believe that this negative thinking which as Mr Kitchin suggested amounted to prejudice, would at least have caused the addressee to regard modification to any of these prior art proposals with considerable reserve if not overt scepticism. This likelihood must, I consider, be given due weight. In my view of the matter, I cannot think that any of the cited prior art would ex facie be likely to have lead the addressee at the relevant date with any enthusiasm to effect the often substantial changes which would bring these proposals within a claim of the Patent: see para 153. My view in this regard is bolstered (but not precipitated) by Mr Dyson's evidence of what actually happened when he tried to interest the industry in Dyson I.

B No ' problem requiring a solution'

157 There is a further general point which follows from the foregoing and it has to do with the problem/ solution approach to obviousness, to which I have referred. First, there is no evidence that from any point of view there was any technical problem at the relevant time with using bags in vacuum cleaners to collect dirt separated from dirt-laden air. There was no doubt scope for higher efficiency, better or cheaper materials to make them of and perhaps other improvements, but no evidence of contemporaneous technical problems, let alone serious ones. There was no question for example, of any long-felt want.

C The experts' evidence on obviousness

158 Regarding the prior art citations, a number of questions were put to the experts (and answered by them) on the basis of vacuum cleaner manufacturers actually wishing at the priority date to develop a 'bagless' vacuum cleaner. In my judgment, this assumption begs the question: the fourth Windsurfing enquiry assumes the court asking itself the obviousness question ' viewed without any knowledge of the alleged invention'. There is no evidence of any wish to scrap the use of bags in the industry at this time and questions addressed on that basis were in my view quite artificial. Even if such a wish existed, there is evidence that there were possible approaches to ridding dirt-laden air in vacuum cleaners of dirt other than by using cyclones eg by electrostatic separation. The weight of some of Professor Allen's evidence on the obviousness issue is for this reason, diminished.

D 'Hang up' etc: A hidden problem?

159 There is a second, more fundamental threshold problem for Hoover on the issue of obviousness and that is the phenomenon of 'hang up' and dog hairs, etc (see para 14). This was the problem which led to the invention of the Patent and is a phenomenon uniquely associated with the use of tapered cyclones in vacuum cleaners. The problem is clearly spelt out in the Patent as the only problem for which a solution is provided. Yet, I can find no reference or even hint of the observation of the phenomenon of 'hang up' etc let alone a recognition that it was a problem with reverse flow (ie tapered) cyclones, prior to the priority date, in any of the evidence, the textbook citations or in the cited prior art. Since prior proposals for the use of cyclones for cleaning air in vacuum cleaners seem on the evidence to have been at best sporadic, being confined to one in-house use (Electrolux) and one document (van Berkel, probably a paper proposal at that) this is perhaps not surprising. It does however in my judgment strongly support Dysons's submission (see para 153, above) that there is absent any reason for taking a step away from the prior art in the direction of the claims. To have done so at the priority date, must be supported by a reason or a motive. And I can find no such motive.

E The Campbell experiments: scaling up, scaling down and amending.

160 The evidence regarding the parties' experimental interpretations of the Campbell machine (see above) also have an impact upon obviousness. As will be seen, this affects not only the Campbell citation but also the rock drilling machine subject of three further citations which have collectively been referred to as 'Johnston/Donaldson'. I have already held that scaling up, scaling down and simple amendment of devices in the vacuum cleaner field are within the competence of the skilled man as part of his common general knowledge (see para 40 above). However, when one is dealing with cyclones, a note of caution must be sounded. The evidence of the experts shows that in this field, scaling up and scaling down does not necessarily yield a predictably proportionate effect in the performance of the device in question. In fact, the matter goes further. The evidence on the Campbell citation (and on other parts of the case) leads me to conclude that it is impossible to predict with accuracy the likely technical and practical performance of a separation cyclone (that is per se and with its associated ducting), merely on the basis of the outcome of a scaling exercise. All sorts of effects peculiar to this art (noticeably, boundary layer effects, wall effects, viscous adhesion, volumetric effects, cross-talk between cyclones, particle size considerations etc) are likely to arise and must be taken into account. In truth, it seems that in a given case, one really has to set up a separator project without a prospect of success, run the appliance and assess its efficacy. All this will apply a fortiori when any serious amendment is made in addition to scaling changes. In this regard therefore, cyclones seem significantly to differ from what I may call 'inactive' mechanical devices such as for example, those in issue in the WORKMATE case, Hickman v Andrews (see para 111, above).To expert and non-expert alike, cyclones seem to have a capricious life of their own.

F The addressee and obviousness

161 In para 33 above, I have held that the addressee is to be regarded as a team comprising qualified engineers having (in essence) experience within the design and development department of a medium-sized vacuum cleaner manufacturer (domestic and/or industrial), and in particular having relevant experience in mechanical and electrical engineering and some knowledge of conical cyclones albeit not of a 'hands on' kind.

162 When faced with the prior art cited by Hoover, the addressee's expertise in cyclones, albeit slight, would be stretched to the limit. An unusual feature of this case is that such composite notional addressee will inevitably be significantly polarised, stretched that is, between ingrained satisfaction with bag technology and by the legal requirement that he should at least take a practical interest in working on the particular citation. Coupled with the fact already noted, that in 1980 there were no technical problems to be overcome in the relevant aspect of the design of contemporary vacuum cleaners and additionally that the 'hang up' problem only became apparent when one actually changed over to separation using conventional tapered cyclones, it will plainly be a difficult case for an obviousness attack to be established. This situation cannot I think, simply be ignored, with the notional addressee team doggedly persisting with an extended programme of non-inventive permutations and combinations to a citation, as Hoover have suggested. On the other hand, it would not be in accordance with authority for the enquiry to be terminated on the basis of an almost instantaneous rejection of a citation as Dyson have in one case (Johnston/Donaldson) suggested. There must of course be a limit to the amount of non-inventive change to a proposal which the addressee may legitimately be supposed to perform for the objection to succeed. Here, in my judgment, reality requires a somewhat abbreviated period of tinkering with a particular citation before its adoption or rejection. As I see it, in this case, prejudice is likely to gain the better of patience rather more swiftly than in some other obviousness cases.

163 I shall next consider each of the items of prior art cited to attack the validity of the Patent on the ground of obviousness.

164 US Patent no 2,171,248 (van Berkel)

72. This patent has a priority date of 1935 and was published in 1939. There is no evidence that any of its embodiments was put to practical use. The patent is entitled "Vacuum Cleaning Apparatus". The devices described were intended for both industrial and domestic cleaning : See col 4, line 17 ('nozzle and pipe') and specifically, lines 40-47. This proposal therefore falls squarely within my understanding of integer (a) of claim 1 of the Patent.

165 Van Berkel has put forward four devices for use as vacuum cleaners. The first two (figs 1-4) make use of a single, conventional conical cyclone for this purpose, the industrial unit being powered by a separate vacuum generating unit whilst the fig 4 unit is a smaller domestic model, with carrying handle. The parties seem to have largely ignored these first two proposals in the light of the two further proposals. Fig 5 is the third proposal for a separator wherein one conventional conical cyclone separator is 'nested' within another so as to be in series with it. This was done so as to achieve a sequential, secondary cyclone separation: Col 3, lines 12-31. Finally, the fourth embodiment shown in figs 7-9 is another separator which is quite different to the others. Here, initial separation is effected in a cylindrical vessel in which dust-laden air is introduced so as to set up a cyclone 'at or near the top of the dust separator housing'. Within this cylindrical housing is a smaller perforated cylinder (75) over which a bag of filtering material (76) is closely fitted to collect the dust from the air being filtered through it. This smaller cylinder, which is connected to an exit filter (66), to suction means and thence to the atmosphere (70), rotates in use and as it does so, a spring-loaded brusher blade (89) sweeps its surface to cause the dust collecting on the surface of the filter bag, to fall to the bottom of the outer cylinder. The inner cylinder also serves to increase the strength of the vortex in the outer cylinder as a result of its rotation. Dust separation is effected both by cyclonic action and by what is brushed off the filter bag. Appended hereto as Annex V are drawings of figs 5 and 7 of van Berkel.

166 Going back to parts three and four of the Windsurfing approach, what is absent from Van Berkel vis a vis the inventive concept is the use of a cylindrical low-efficiency cyclone upstream and in series with a high efficiency cyclone having a frusto-conical part. Hoover have asserted that it would be obvious to replace the outer conical separator of fig 5 with an outer cylinder separator cannibalised as it were, from fig 7. The reason they give for making this change is that the resulting vacuum cleaner would be cheaper to manufacture: Allen Report, para 169.

167 The evidence shows first of all, that van Berkel's proposals did not on the whole much impress either expert. Thus Prof Syred in cross-examination (2/154-155):

75. And Prof Allen in cross-examination on fig 7 (4/486):

168 With regard to the Fig 5 embodiment, the experts were agreed however that from their perspective in the cyclone field, this would have been the embodiment which would have attracted their attention, as it was based on conventional wisdom of using frusto-conical separators, this time in series. Under cross-examination, Prof Syred said this (3/289-290):

82. On the same point, Prof Allen said (4/489-490) that the fig 5 embodiment would be the starting point of the van Berkel patent because of the attraction of the hydraulic separation between the cyclones.

85. On page 492, Prof Allen added " Having made it, I think it stands a fair chance of working."

169 With regard to the Fig 7 embodiment, both experts were agreed that this represents 'an integrated proposal': see 2/291 and 4/486. In other words, it is a containment vessel specifically designed to accommodate the rotating filter in the middle. The two were designed to work together. Prof Allen was concerned that though it might be more efficient, the fig 7 embodiment would be likely to break down and "For this reason one would not build it". On page 4/496 Prof.Allen said this:

170 Even without invoking the ongoing prejudice within the industry in favour of bags, it seems plain that there is no reason technical or commercial why even 'the Allen man' (cf para 32, above) that is- would have made the 'swap' in outer cyclones between van Berkel's third and fourth embodiments which Hoover suggest and which would have brought the result within the scope of the claims of the Patent. Quite the contrary: the fig 5 proposal would have seemed the best option in the document- and, so it seems, would be likely to work. The fig 7 embodiment on the other hand was an 'integrated concept' and moreover would have been thought to have been less efficient and not attractive to build. All this is of course comes from the perspective of an addressee whom I have held to have been rather too well qualified. Consider now the prospect from the point of view of the skilled man not in possession of much cyclone expertise. One has necessarily to speculate, but it seems to me that at least on the basis of 'two better than one' it would be reasonable to suppose that he would have gone for the Fig 5 embodiment as well.

171 But that is only the beginning of the enquiry. Having built the fig 5 machine, there remains the further questions of whether 'hang up' would have been observed and would have been regarded as a problem and vitally, whether the skilled man would have come up with a relevant solution. To short cut all this and seek to justify the swap on the basis merely of a saving in manufacturing cost seems to me quite unreal. In my judgment, Hoover have failed to prove their case on obviousness in the light of van Berkel.

US Patent no 2,678,707 (Campbell)

172 I have already considered Campbell in some detail under lack of novelty and have held that to the notional addressee reading it at the priority date, the separation chamber (2) would not appear to be acting as a cyclone at all, even when the tangential entry option is chosen. A major portion of the Dyson claims are thus missing even assuming that this device is indeed a vacuum cleaning appliance as I have construed the phrase (which it is not).

173 On the evidence, it is difficult to assess what the likely reaction of the notional skilled addressee would be on being presented with Campbell at the priority date. Given its context in the food industry (or in so-called 'agri- business'), its essentially industrial context, its age and the prejudice in the industry in favour of bags, I sense that the addressee would spend little time with Campbell as a serious candidate for some wholly new development in the vacuum cleaner field. The preparation of spray-dried milk and eggs has never I would guess, been a bonus to those manufacturing vacuum cleaners. Hoover argue however that the scaling down of Campbell to 'domestic' proportions would have been well within the competence of the skilled man at the relevant time. Maybe so, but I can find no evidence which would motivate him to do so with a reasonable expectation of a dividend. Moreover at the same time, the skilled reader would need to work hard so as to arrange for the dirty air inlet velocity to be such that

174 Even if the skilled man were to take an interest in Campbell, its teaching is a very long way away from anything claimed in the Patent. Professor Allen as one would expect, was cross-examined concerning the necessary modifications which would be required to bring Cambpell within the claims: 4/567-568. His answers were however based on the belief that Campbell in fact disclosed all the features of claim 1 – which I have held not to be so. As with other aspects of their obviousness case, Hoover were not helped by the absence of any witness who could speak with authority as to current thinking in the vacuum cleaner field in 1980. For reasons already given, I do not regard the evidence of either expert witness to be of primary assistance on this point. In summary, in my judgment, the Patent is not obvious in the light of Campbell.

[Johnston/Donaldson]

175 I have already considered this group of citations under 'lack of novelty' and as with Campbell, I will try to avoid repetition. Under the ground of obviousness, I have however decided that it is legitimate to consider these three documents together even though there is no cross-reference to the Bulletin in the other two citations. I have done this because (to use the language of the EPO) there is I believe, a 'logical link' between them all.

176 I have held that save for integer (a), the remaining integers of claim 1 are present in these citations. The difference between the alleged invention and the Johnston/Donaldson roof drill dust collector is therefore solely one of description and thus, application.

177 In connection with the last stage of the Windsurfing enquiry, Hoover rely in particular upon the passage in the Bulletin which reads as follows:

178 In approaching obviousness, one must in my judgment-and contrary to Dyson's submission, first assume a degree of technical interest in the Johnston/Donaldson apparatus on the part of the addressee at the priority date, however 'bagridden' his thinking might have been. He would however immediately note that this six wheeled behemoth was not as described, even remotely ready for modification so that it could be used as a 'vacuum cleaning appliance'(see above, para 46) in which he would have an interest. This is an item of mining equipment used to restore dust-laden air generated by abrasive underground roof-drilling operations, to something like a breathable condition. Furthermore, this is not a cleaner. It is a proposal for a large vacuum-operated air purifying system which is part of a mobile underground drill. Overall, it is actually a dust generator and not a dust abstractor. The addressee would also gather that this machine ingested mineral particles and rock dust and not the variety of materials likely to be encountered in industrial, commercial or domestic cleaning operations. He would indeed note that bags are not used to collect the drillings. He would notice that the pressure drops which are required to draw the drilling dust away from the drilling bit into the separators were by domestic and industrial vacuum cleaning standards, quite titanic. He would read that the second-stage, side arm cyclones in the Johnston patent(s) were put forward as an being an improvement on the conventional cyclones used at the same place in the earlier dust collector to which both the Bulletin and the Manual relate.

179 As a starting point, the addressee would have to begin by first trying to build a suitably modified version of one of the Johnston/Donaldson dust collectors described in the cited documents. He would realise that if anything practical was to be made of this proposal in the industrial and domestic vacuum cleaning field, a drastic and comprehensive change of scale, vacuum pressure, ducting etc would be immediately be required, together with substantial amendment to the design itself (eg in dust collection, mobility etc) so as to render it suitable for this totally new use. Moreover, he would feel fully justified in making liberal changes to the size and nature of the cyclones proposed by Johnston because of the very special purpose for which they were originally designed:

181 On the other hand he would be aware that the reason for the selection of these types of cyclone was to enable them conveniently to be used within the confines of a mineshaft. Johnston/Donaldson wished to maximise the trade off between capacity and height, that is maximum collection capacity within the smallest vertical space. This applies equally to the cylindrical, first-stage cyclone as well. 'Compactness' as applied to this machine is of course unrelated to the sort of compactness required of a vacuum cleaner.

182 This exercise in adaptation and miniaturisation would in my judgment involve a substantial research programme for inter alia, the following reasons:

86. The Professor also mentioned that practical information is missing from the disclosure to assist the reader in this respect.

183 In my judgment, if anything went wrong with the first stage cyclone in a programme of 'adaptation and scaling' of Johnston, the addressee would be likely to scrap the cylindrical cyclone of the first stage altogether and replace it with an efficient conventional, tapered cyclone of the kind he knew about and additionally perhaps, on the 'two is better than one' approach. Since this cyclone was chosen by Johnston/Donaldson with primary regard to height limitations, specialist performance ('Coarse Dust') and compactness, its configuration out of context in a mine would be unlikely to appear critical anyway. And were he to do that, the addressee would just be at the threshold of the Patent since then and only then, might he have noticed the 'hang-up', dog hair/fluff phenomenon, which is the problem which the Patent addresses. Then to decide again to go back to a cylindrical first stage cyclone, re-designed moreover to overcome the vortex finder problem, would in my judgment not only be most unlikely, but were it to be done, it would I consider, be inventive.

184 In his evidence on the Johnston patent, Prof. Allen said (4/501):

92. This is really the high water mark of Hoover's evidence on this aspect of Johnston/Donaldson and in my judgment, even without the evidential matters to which I have referred, is not enough to carry their case on obviousness. Prof Syred was even more forthright :

95. The attack based on this citation must in my view, also fail.

96. Subsidiary claims

185 Since the attack based on lack of novelty of claim 1 has failed, so also does the lack of novelty case on these dependent claims. In any event, the features of claims 2,4 and 5 are not even arguably present in van Berkel's Figs 5 or 7 and the features of claims 4 and 5 are not present in Campbell. The only issue on the subsidiary claims is therefore obviousness.

186 Claim 2 it will be recalled, brings in the feature of what I have called the 'nesting' of the cyclone units ( ' the high efficiency cyclone being positioned within the lower efficiency cyclone unit.'). I have held that cyclone 'nesting' was not within the common general knowledge. Of this feature, the Johnston/Donaldson documents contain not a hint. The additional problems of further adapting this proposal to a 'nesting' configuration were pursued with Prof Allen in cross-examination on 4/527-529. It was agreed that the multi-cyclone proposed in the prior art document would not fit within the first stage cyclone. The Professor then suggested either fitting in two small multi-cyclones instead or rather, change to a single conventional cyclone. No reason was advanced which would drive the addressee to effect this change. All this would of course, be in addition to the numerous other changes to which I have already referred. In my judgment, this evidence merely shows that in order to get this citation within claim 2, hindsight, hopeful persistence and a major research program would be required. This attack fails.

187 With regard to van Berkel, Hoover's obviousness case in effect required a change to bring into 'nesting' combination, the relevant parts of Figures 5 and 7. Their case on claim 2 is thus essentially the same as for claim 1 and for the same reason, it fails. According to Hoover, Campbell is a 'nesting' proposal for a vacuum cleaning appliance and renders the Patent invalid both on the grounds of lack of novelty and obviousness. I have held that this is not so and the reasons which I have already given apply equally to claim 2.

188 With regard to the independent validity of claims 4 and 5, not much was said. I have held that their distinguishing features, particularly that of the appliance being 'upright on wheels' and having a rotating motor driven brush were features of the common general knowledge. If I had held the antecedent claims to have been invalid, the added features of these claims would not in my view have saved them from invalidity on this ground.

189 Dyson did not plead commercial success to buttress its response to the attack on the Patent based on obviousness. It did however rely on what it called 'Technical Acclaim' and in this connection, pointed to a number of facts, including Hoover's own allegedly inconsistent commercial attitude to vacuum cleaners incorporating cyclonic means to rid the air of dirt. It pointed out (correctly) that these facts were not the subject of cross-examination. Dyson suggested that I could legitimately take into account the 'technical acclaim' accorded to Mr Dyson's invention inter alia in relation to my approach to the issue of obviousness. I reject this argument; this is 'back door' commercial success. In modern practice, the rules regarding the reliance on commercial success in patent infringement actions however raised, are subject to specific procedural requirements: CPR, Part 49, para 9.3. I wish to record therefore that Dyson's submissions under this head have not been an ingredient of any of my findings in this action.

190 Section 14(3) of the Act provides:

97. Section 14(5) requires that the claims shall inter alia '(b) be clear and concise.' Section 72(1)(c) provides that a patent may be revoked if

191 Hoover have attacked the Patent on this ground on the following pleaded basis:

98. It was accepted that sufficiency to the person skilled in the art should be judged as of the date of application for the Patent.

192 Hoover's objection was therefore not that what was described in the Patent could not be built or operated. Mr Floyd suggested that their purpose in raising this plea was, as often happens in patent infringement cases, to 'squeeze' the patentee on construction and thus, infringement. In essence, the objection is directed to the 'in series' point previously considered (see para 65, above). Hoover argued that if the claim included within its scope methods of connection between the cyclones which are other than 'one following the other' directly, then the specification contains no teaching as to what those are.

193 I would mention that Hoover's initial case on insufficiency at trial was not put as it was pleaded, as may be seen from Prof Allen's Report, paras 172-176. His argument seems to have been that a skilled man could not build a HTV appliance from the information given in the Patent. As this was not put to Professor Syred in cross-examination and was not argued in this way by Mr Floyd in closing, I do not propose to make a finding on it.

194 The parties were agreed that the relevant principles of law to be applied are those laid down by the House of Lords in Biogen v Medeva [1977] RPC 1. It was common ground that this authority, which was a 'chemical' case, applied to mechanical patents such as the patent in suit. My attention was also drawn to Terrell (15^th Edn), paras 7.44-7.47, as representing a correct epitome of the relevant law and in particular to Lord Hoffmann's speech in Biogen at page 49 of the report where he said:

99. Though this passage was dealing with sufficiency in the context of enablement the same reasoning applies to a case of insufficiency under s. 72(1)(c): see page 53, lines 20-27 of the report. In essence, to be valid under this head, a patent must be sufficient across its full width. Dyson submitted that claim 1 defined with sufficient precision a class of vacuum cleaning appliance sharing a common principle of operation namely, that defined in similar terms to the terms which I have set out in para 28 above as representing the 'inventive concept'. I agree with this submission and accordingly the objection fails.

Conclusion

193. In the light of the findings recorded above, the action succeeds and the counterclaim will be dismissed. I shall hear Counsel on the appropriate form of order to be made in due course.