made from glass fibre or textile fibre and sewed, glued or spot welded. These operations are labour intensive and
expensive. The individual filter pockets bulge under operational conditions, resulting in uneven flow conditions and
allowing the pockets to touch each other, thus impairing their filtering efficiency. Fluttering motions occur causing
unsatisfactory performance and damage to the pockets because of the loosening of fibres. Spacers were used to
prevent bulging but the known method of fixing these was timeconsuming and caused a reduction of the active
filter surface.
Against this background the specification sets out the object of the invention in what can be called the
promissory clause:
"The object of this invention is to provide a pocket filter element which attains the performances of rigid high surface filters
as regards the degree of dust separation and dust storage capability while avoiding the aforementioned disadvantages."
The socalled consistory clause follows:
"This object is achieved according to the invention by a gas filter element comprising a supporting frame and a plurality of
wedgeshaped filter pockets undetachably connected with this frame, and characterised in that the pockets are of a suitable
filtering medium and have selfsupporting properties provided by a continuous, edgetrimmed weld and spacer elements
secured to the filter medium in the air flow direction and, if desired, by additional welded stiffening zones, and that the edges
approached by the air flow are continuously and
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undetachably connected with the supporting frame which is made of a hard foamed material and is connected directly with
the filtering medium by a foaming process."
The specification explains the invention with reference to two drawings and concludes with a more specific promise:
"The advantages achieved by the filter element according to the invention reside especially in the fact that by its application
the filter can be rapidly exchanged, even by untrained labour, while ensuring that break through places of dust are avoided.
The filter pockets do not tend to flutter and the filter element achieves the performance of rigid high surface filters as
regards its dust separation and storage."
There are five claims. Claim 1 conforms with the consistory clause. In the quotation that follows the numerals in
brackets (according to the German tradition of claim drafting) are references to the drawings. The enumeration by
way of letters represents the division of the claim into integers proposed by the parties and adopted by the court a
quo. The claim reads:
"1.
(a)
A gas filter element
(b)
comprising a supporting frame (2) and
(c)
a plurality of wedgeshaped filter pockets (1)
(d)
undetachably connected with the frame, characterised in that
(e)
the pockets are of a suitable filtering medium and
(f)
have selfsupporting properties
(g)
provided by a continuous edge trimmed weld (3) and
(h)
spacer elements (4,6)
Page 244 of [1998] 1 All SA 239 (A)
(i)
secured to the filter medium in the air flow direction,
(j)
and, if desired, by additional welded stiffening zones (7),
(k)
and that the edges approached by the air flow are continuously and undetachably connected with the
supporting frame
(l)
which is made of a hard foamed material
(m)
and is connected directly with the filtering medium by a foaming process."
I do not propose to deal with claims 2, 3 and 4 because they are dependent upon and narrower than claim 1 and
therefore do not contribute to the resolution of any issue on appeal. Claim 5 should be mentioned. It is a socalled
omnibus claim and claims
"5. A gas filter element, substantially as described herein with reference to the accompanying drawings."
The description of the invention with reference to the drawings differs from the description of the invention in the
consistory clause but these differences need not be dealt with at this stage of the judgment.
Reverting to claim 1, the meaning of "selfsupporting" was the subject of much evidence and debate because of
its importance for the decision on both infringement and invalidity. It is an ordinary English word with no special or
technical meaning. Evidence, expert or otherwise, is impermissible to explain it. According to the Shorter Oxford
English Dictionary it means "(of a physical object) not requiring the usual support". "Support" has many shades of
meaning but in the context of the invention the following are apposite: "1. To bear the weight of, especially from
below. 2. To hold in position; prevent from falling, sinking or slipping. 3. To be capable of bearing; withstand".
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