Note: Descriptions are shown in the official language in which they were submitted.
lOS6031
Background of the In~ention
l. ~Fie~d of the Invention
. .. . .. . . . . . . . . . . . . . . .
This invent~on pertains to resilient, self-aligning,
electr~cal connectors having electrical contacts made of metal-
filled or carbon-filled, resilient, elastomeric layers inter-
posed between non-conductive elastomeric layer~. The invention
particularly pertains to methods for maXing layered elastomeric
structures used to electr~cally connect t~o or more sets of
- electrical conductors proximately positioned in a one~to-one
lQ relationship, each set consisting of a plural~ty of closely
spaced conductors positionally fixed with respect to each other.
2. Description of the Prior Art
Prior art connectors for electrically connecting two
or more sets of electrical conductors such as tape cable con-
nectors, plug~tn printed circuit board connectors~ integrated
circult connectors, liquid crystal display unit connectors and
; the like usually include compllcated assemblie~ that have complex
metal contacts for complettng the electrtcal ctrcuits, Some
connectors ~nclude sharp-polnted contacts that are forced through
insulation or insulating films bending, scratching and stressing
the conductors to provide adequate electrical contact. Charac-
teristic of most prior art de~ices are complicated electrical
eon~acts in the form of ramps, rings, fingers and t~e like
made of springy metal material which maintain engagement with
the conductors by mean~ of elastic deflection, These types
of electrical contacts are usually expensive to make and diffi-
cult to assemble into a connector. Additionally, they have the
. 2 - ~
, ~
1056~3~
disadvantages of being generally di~ficult to reproducably
fabricate and when fabricated, occupying an undesirable amount
of volume and subject to fat1gue when under continuous use.
Where two or more sets of electrical conductors are to
be connected to each other, each set consisting of a large
number of very small conductors closely align~d next to each
other, the electrical contacts must in some measure assure
exact alignment of ~he conductors so $hat each conductor of a
first set w~ll contact only with the correct corresponding
conductor or conductors of a second set. This alignment is
generally achieved by means of spaced aperturesin the connectors
that contain corresponding contacts, Where a large number of
contacts are so situated or where repeated making and breaking
of the contacts is experienced, misalignment, wear, bending,
shorting and other types of circuit failure are commonly exper-
ienced. Moreover, permanent or semi-permanent electrical -
connections of this type are undesirable or impossible. The
metal to metal contacts expertence surface abrasion due to the
wiping,action ~f the initial contact which, in time, corrodes :
~0 thereby increaslng the contact resistance. The actual contact- 1,
ing area of a metal to metal contact is typically less than one
thousandth of the total surface area 'of the metal contact. If
permitted, moisture and hostile atmospheres can migrate between
the contact surfaces rapidly deteriorating the quality of the
electrical contact.
These dlsadvantages can only be overcome ~y providing a ¦ -
connector which ts soft and resilient and which sealingly engages
the contacts to be connected yet whlch presents su~stantially
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1056031
no surface abrasion during initial contact. It is therefore
an object of thls invention to create such a connector
which is very simply and reproducibly fabricated from
known materials by methods which are susceptible to
u~ilization of low skill level manpower, low volume economy
of scale and negligible waste of materlals.
Summary of the Invention
The present invention relates to a method of
making a layered connector element for electrically connect-
ing sets of spaced electrical conductors comprising thesteps of: A. assemblying alternately ~n parallel relation-
ship sheets of electrically conductive material and sheets
of elec~rically non-conductive material, into a block
~tructure, B~ slicing fro~ the block, in a plane perpendic-
ular to the planes of the sheets, a slab containing,
alternately, elongated elements of electrically conductive
ma~erial and elongated elements of electrically non-conductive
~aterial, and C. slitting from the slab, in a plane to
which the elongated elementa of the slab are substanti~lly
2~ ~ normal, a layered connector, the linear dimension of the
connector along a direction perpendicular to the layer6 and
passing therethroug~ being at least several times the largest
linear dimens~on of any single layer.
In its apparatus aspect, the invention is useful
with a system electrically connecting at least two sets of
Rpaced electrical conductors, and comprises a strip of
~ub8tantially parallel alternate layers of conductive and
non-conductive cured elastomer~ the linear dimension of the
strip along a direction perpendicular to the layers and
30 pa6Bing therethsough being at least several times the largest
l~near di~ension of any single layer,
A layered strip connector according to this
invention can be produced by any of several methods, although ;
~b/~`b_ ~ 4
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certain me~hods are pre~errcd over others due to economies
of scale, adaptability to automation, uniformity and
quality control. Generally, a sheet of non-conductive
elastomer is sprayed, cast, molded, extruded or calendered
and partially or fully cured. A sheet of conductive
elastomer is sprayed, cast, molded, extruded or calendered
on top of the previous sheet, or sprayed, cast, molded,
extruded or calendered separately and placed Dn top of
the previous sheet with any necessary binder included. The
process of placin~ conductive sheets on top of non-conductive
sheets is repeated many times to form à block consisting
of a stack of ~heets of an appropriate height. The stack of
sheets is then post-cured to effect a binding between all
the sheets. The stack is then sllced, approximately perpen-
dicular to the sheets, to form slabs containing alternating
elongated elements of conductive and non-conductive mater~al.
The ~labs are thenislit in planes to which the elongated
elements are substantially normal to form a layered connector ~ --
according tD thi~ invention.
A connector made accord~ng to this invention
consists essentially of layers of electrically conductive,
elas omeric resin (which can be made conductive in any
~nown conventional manner) and non-conductive elastomer~c
resin-alternately interposed to form an electrical connection
between two or more sets
- ', '
.: "
,~
':
~ mb/ ~ - 4a -
,, , . . . . , ~ , .. . .
1056031
of proximately spaced electrical conduc~ors. The electrical
connector element exlsts independe~tly of the sets of conductors
as a strip of resilient mater~al co,nsist~ng of a sertes of metal-
filled or carbon-filled, elastomer~c resln layers interposed
between non-conductive resin layers, the conductive layers
forming the electrical contacts of the connector element,
Generally, the number of layers per unit length of the c~nnector .
strip will be selected such that at least one conductlve layer
and typically a plurality of electrically conductive and non-
conductive layers contact each conductor as well as each space
between adjacent conductors of any set. Since the number of
layers is typically large in comparison to the number of conduc-
tors in any gi~en situation, the connector ef~ec~s a self-aligning
function by permitting electrical contact only between corres-
ponding conductors.of two or more sets connected. The layers
are subs~antially parallel to each other ana are approxtmately
perpendicular to.the surface of the conductors contacted, The
layers need not be of the same thickness and in some applications ~:
particular thicknesses for the conductive and/or non-conductive .
layers can be advantageously established. In general, the linear
: dimension of a layered connector perpendicularly transverse to
the layers forming the connector is at least se~eral and typioally
10 to 100 times the largest ltnear dimens~on of any ~ingle layer
forming the connector.
Elastomers which can ~e sat~sfactorily used ~nclude - i
copolymers of butadlene-styrene, butadiene-acrylonitrtle, and
butadiene-isobutylene as-well a~ chloroprene polymers, poly-
sulfide pol~mers, plasticized vinyl chloride and vinyl acetate
polymers and copolymers, polyurethanes and silicone rubbers.
. The ~ilicone rubbers conventionally are dimethyl, methyi-phe~yl,
: - 5 -
.
1~56~31
methyl-vinyl, or the halogenated slloxanes that are mixed with
fillers such as a silica to impart proper rheology and vulcanized
or cured with peroxides or metal salts~ Silicone rubber is
generally preferred because of its aging characteristics and
its retention of physical characteristics at temperature extremes.
The elastomers used should be form stable when partially cured,
that is, they should not deform unduly under their own weight,
nor should they plastically deform after curihg, but rather
should be resiliently renitent.
It is, therefore, preferable that the connector consist
only of alternating layers of conductive and non-conductive
elastomeric resin, Greater integrity (i.e. unitary nature of
the elastomeric material) can be assured by using the same elas-
tomeric material for both the conductive and non-conductive layers,
the differences in conductivity resulting only from the choice
of appropriate fillers.
A non-conductive elastomer is an elastomer having a
volume resistivity equal to or greater than 109 ohm-cm. While
the res~stivity of the conductive layers can be varied over wide
ranges, typically 10-4 to 104 ohm-cm., low resistivity values
are preferred to reduce problems such as thermal dissipation
and capacitive interference, which can be experienced at the
higher resistivity values.
The preferred elastomers for use in both the conductive
and nonsconductive layers are the silicone rubbers, to which may
have been addea fillers to enhance thelr handllng properties.
Examples of non-conductive silicone elastamers are General
Electric Company RTV-615 and Rodhelm-Reiss Compound 4859
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1056031
Silicone elastomers typically in the absence of co~ductive
fillers, have a volume resistivity of 10 4 to 1015 ohm-cm.
and a dielectric strength of about 500 volts per mi1 in a
one-eighth inch thick sample.
Conductive elastomers ha~ing higher values of
resistivity, 10 to 104 ohm-cm., are generally created by
using a carbon-filled ~lastomer. An example of a carbon-
filled conductive elastomer is Union Carbide silicone
compound K-1516.
Conductivé elastomers having lower values of
resistivity, 10 4 to 10 ~hm-cm., are created by incor-
porating into the elastomer conductive fillers such as
copper, nickel and silver, and metal-coated fillers such
as silver-coated copper and silver-coated glass. The
metal-filled elastomers may also contain carbon to improve
the physical characteristics of compression set and
strength. An example of a metal-filled conductive elastomer
i s: , .. ... .
TABLE 1
- !
Material Wei~ht ~ -
'.
Silicone rubber compound -
methyl phenyl vinyl s~lDxane gum ~ ~-
(General Electric, SE-5211U) 13.0% ;~
2,5-bis (tert-butylperoxy)-2,5-dimethyl-
hexa~e carried on inert carrler, 50% active
(R. T. Vanderbllt Co., VAROX, a trade mark) 0.1%
Dicumyl peroxide carried on carrier of
precipitàted calcium carbo~ate, 40% active
~Eercules, Inc., Di-Cup 40C, a traae mark) 0.1%
Silves powder
Average particle diameter, 0.6-3.0 microns
Apparent density, 8-16 gmslin3
(Handy & Harmon, SILPOWDER 130, a trade mark) 63.8%
Silver powder
Average particle diameter 3.0-4.0 microns
Apparent density 16-19 gmslin3
(Metz Metallurgical Corp., EG-200) 11.5%
mb/~ - 7 -
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Silver flake
Average particle d~ameter 10.0 microns
Average particle thickness 1.5 microns
Apparent denslty 20-27 gms/in3
(Metz Metallurigcal Corp., Ag Flake #6) 11.5%
c~ tr~e r~4~
Examples of other conductive and non-conductive elastomers
usable in this invention are to be found in U.S. Patents 3,140,342;
3,412,042; 3,609,104 and 3,620,873.
The resilient character of the elastomers involved assures
a good electrical connection with the conductors by elastically
deforming in response to external forces such as would be experi-
enced upon insertion of the c~nductor set. This effects a
vibrational absorbing and cushioning not a~ailable from undamped
flexible metal connectors. This damped flexible supporting of
the surface of the conductors also hermetically seals the con-
ductor surface after contact has been made thereby inhibiting
corrosion by preventing the mlgration of hostile fluids to the
contacting conductor surface~ The connectors of this invention
are easily reproduced over a wide range of contact resistance~
hardness, layer th~ckness and other mechanical and electrical
~ariables,
While the thickness of the layers can be varied sub~
stantially depending on the ind~vidual demands of the parttcular
situattonr for opt~mum design the layer t~icknesses should Pe
chosen so that there are as many conductive layers ~er un~t
.length of the resulting connector element as possible,wh~le
simultaneously avoiding any electrical malfunction caused ,by the
proximity of the adjacent conduct~ve layers under the ntended
conditions of use. While satisfactortly perform~ng la~ered
strlp connectors can be made with elastomer la~ers as thin as
.0003 ~nches and as thick as ,125 inc~es, from practtcal con~
siderations of quality, ease of assembly, economy~ etc,~ the
-- 8 --
1056031
layers need be no greater t~an ,040 ~nches and ~hould pe ~o
thtnner than .001 inches . A one~to~one correspondence between
the conducttve layers of the connector and the cQ~ductors of 'i
one set of conductors may be desirable ~n particular s~tuations,
In all situatlons the lndtvidual la~er a~mensions are chosen
with regard to the final conne~tor d~me~s~ons such that ~he
. llnear dim-engion of the co~nector perpend~cularl~ tran~ver~e to
the layers forming the connector i8 at least several times the
largest linear dimension of any single layer form~ng the con~ -
nector. ~.
The layered connectors made accord~ng to th~s ~nvention
can have several configurations, In one embodiment, the connec-
tor can oomprise simply a strip of substanttally parallèl alter- j
nate layer.s of conductive and non-conducti~e cured elastomer, ,.:
the linear dimension of the str~p perpendicularly transverse to
the layers fonming ths strip be~ng at least several t~mes the.
largest linear dimension of any single layer present ~n the
strlp, In another embodiment, a plurality of the strips can be
combined with means for retaining the strips ~n substantially.
2b fixed relation to one another, One such retain~ng means is formed
from antipodally bordering lamina cured or vulcan~zed to the
original block of alternately layered conductive and non-con-
duc.tive elastomers. Other retaining means may be formed in-
dependently of any shape, subject only to conventional choice
. of design, A strip ~f substantial~y parallel layers of conduc~
ti~e and non-conducti~e cured elastomer is then bonded to the-
peri,phery of the retalning means, Al~ernat~ely~ sla~s o~
elongated elements cut from a block of alternately layered
1056031
elastomers can be bonded to a central core body. The central
core body with the bonded layered elastomer slabs can then be
cut in planes perpendicular to the elongated elements comprising ,
the slabs into individual connectors.
In the broadest sense, the invention comprises means and
method for making a means for connecting sets of spaced elec-
trical conductors compr~sing recurrent, substantially parallel
layer,s of conductive and non-conductive material bonded together
in a unit. Particular features and advantages of the invention jl,
will become apparent from the following description in conjunc-
tion with the preceding summary, the accompanying drawings and ,~
claims.
Brief Description of the Drawings ~,
Figure 1 is a sectional view of a layered strip connector
according to this invention~
Figure 2 is a perspective view of a block and a slab
' sliced from that block formed in making a connector according
to this invention.
Figure 3 is an enlarged detail perspective of the slab
of Figure 2 and a connector element slit from that slab.
Figure 4 is an explodea perspective view of an elec-
trlcal apparatus being connected to a printed circuit board
with a connector according to this invention.
Ftgure 5 is an elevation view of a connector having
retaining means formed of antipodally bordering laminae accord-
ing to this in~ention,
.
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1~56~31
Figure 6 is a perspective view of a block and a slab
sliced from that bloc~ having antipodally ~ordering laminae
according to this invention~
Figure 7 is a schematic representation of a method of
forming blocks according to this invention.
F~gure 8 is a perspective view of a body with a plurality
of slabs of elongated elements bonded thereto.
Figure 9 ls a perspective view of a connector a~cording
to this invention cut from the body illustrated in Figure 8.
Descri tion of the Preferred Embodiments
P
As shown in Figure 1, a layered strip connector 10 contains
alternate layers of conductive elastomeric resin 12 and non-
conductive elastomeric resin 14 bonded together to form a unitary
structure. Surfaces 16 and 18 are suitable for contacting sets
of approximately positioned conductors (not shown) for elec-
trically connecting the sets of conductors, Electrical conduc-
tion can take place in either direction, between the surfaces
16 and 18, through the layers of conductive elastomer 12, while
substantially no electrical conduction takes place through the
layers of non-conductive elastomer 14. The individual conductive
layers 12 are therefore insulated from each other.
A connector according to this in~ention is constructed
by assemb~ying by molding, casting or some other method a plurality
of sheets of conductive and non-conductive elastomers alternately
to form a layered b70ck 20 shown in Figure 2. The block is cured
sufficiently to ensure physical integrity of the block so as to
prevent any layer separation at any subsequent step in the manu
facturing procedure or dur~ng use. The cured block 20 ~s ~hen
sliced in a plane perpendicular to the planes of the individual
sheets ~orming the block to provide slabs 22.
105603~
The slabs 22, shown in more detail in Figllre 3,
consist of a plurality of rods of conductive elastomers
24 and rods of non-conductive elastomer 26, bonded
together. The rods of conductive elastomer 24 are con-
ductive not only through the thickness of the slab 22,
but also longitudinally through the length of the
conductive rods 24. The slabs 22 are then slit perpen-
dicular to the rods 24 to form the connecting strips 10.
The strips 10 can be used either individually or in
combination with other similar strips to form layered
connectors according to this invention. In generàl,
the linear dimension L of the strip 10 perpendicularly
transverse to the layers 24 and 26 forming the strip 10
is at least several times and typically 10 to 100 times
the largest linear dimension of any single layer 24 or 26. -~
The assemblying of the sheets of electrically
conductive and non-conductive material into a block may
be performed by several different methods. An example
of the production of a block ready for slicing containing ~
carbon-filled silicone rubber for the conductive layers -
is as follows: a plurality of insulating sheets 2 in. x
4 in. x .~10 in. were produced from a Rodhelm-Reiss Silicone
. . .
Co~pound 4859 catalyzed with 1% Verox (a trade mark) by
pressing for one minute at 340F. until partially cured.
Conductive sheets 2 in. x 4 in. x .010 in. were produced
from Union Carbide Compound K-1516 catalyzed with 1%
Verox (a trade mark), pressed for one mlnute at 450F.
until partially cured. The conductive and non-conductive
sheets we~e stacked alternately to produce a block 1/4 in.
hlgh, Four such blocks were stacked to form a block 1 in.
high. This block was cured in a press for one-half hour
at 450F. and post-cured without pressure for 4 hours
at 350 F. -~
mb/~o - 12 -
1056031
The block was then sliced into slabs 2 in. x 1 in. x ~100 in.
The slabs were then sl~t into connecting strips 1 in. x .050 in.
- x .100 ln.. Each layer within the connecting strip had linear
outside dlmensions of .010 in. x .050 in, x .100 in. and tne
diagonal linear dimension through the center of the layer was
calculated to be approx~mately .11~ in. The linear dimension
of the connecting 8trip perpendicularly transverse to the layers
forming the strip (l--ln.) is, therefore, at least several t~mes
the largest linear d~mens~on o~ any single layer ~ ,112 in . ) .
An example of the production of the layered connector
containing silver filled silicone rubber is as follows: sheets
of non-conductive sil~cone elastomer 2 in. x 4 in. x .010 in.
were produced in the same manner as in the previous example.
Layers of conductive silicone elastomer were produced w~th the
formulation set forth in Table 1, ~len~ed and pressed into uncured
layers 2 in. x 4 in, x ,010 in. The conductive and non-co~ductive
layers were alternately stacked to produce a block 1/4 inch high.
Four such blocks were`stacked to form a block 1 inch high. This
blocX was cured at 450~. for one-half hour in a press and then
post-cured without pressuse ~or 4 hours at 350F. Thls block
was sliced in a manner s~milar to the previous example to form
connector strips 1 inch by .050 inches by ,100 inches.
Blocks of sheets suitable for slicing into connectors can
: als~ be produ¢ed by fully curing the conductive and non~conduotive
~25 sheet~ of t~e foregotn~ examples sep æ ately, interleafing the
sheets of conductive elastomer with those of the non-conductive
elastomer with a curable3adhesive therebetween, and subsequently
curlng under Fressure
- 13 -
; 105~031
Blocks may also be produced by casting a layer of non-
conductive elastomer and partially cuxing that layer, casting
a layer of conductlve elastomer onto the non-conductive layer
and partially curing the second layer, continuing to cast and
cure alternate layers of conductive and non-conductive elastomers
until formtng a block of the destred dimension and finally curing
the block to ensure that the sheets do not separate. This method
may also be used with molding rather than casting.
Another method for producing blocks of sheets suitable
for slicing into connectors, according to this invention, com-
prises extruding long continuous lengths, typically up to 800
feet and 3 to 5 inches wide, and partiaily curing the long con-
tinuous lengths of non-conductive elastomer to a state that it
is easily handled and does not deform under ttS own weight,
The long continuous strip of non-conductive elastomer shown in
Figure 7 as 60 is then used as a base or substratè upon which
a continuous layer of electrically conductive elastomer 62 can
be extruded or calendered as a continuous layer. The contlnuous
layer of conductive elastomer 62,along w~th the strip of non.
c~nductive elastomer 60 is then wound on an octagonal or other
similarly shaped drum 64. The drum 64 has sldes 66, typically
, " about 10 inches in length. The thickness of the layers 60 and
~ ~ O-V~O
~62 ls typically 4.~a~ inches. The drum 64 is rotated in t~e
~- ' ' direction indicated by the arrow until 50 to 100 layers of the
con,ductive and non-conductive elastomer are wound one on top
of the other. The drum 64 is then stopped and the multtple
layers 68 formed on each surface 66 are removed from the drum ,,
6~ by cutting generally along lines 70. The multtple layers
68 are then cured under pressure, either individually or stacXed
together to result in a block -20 as shown in Fiqure 2. Thls
- 14 -
1056C~3~
block can then be sliced tnto slabs 22 and then slit into layered
strips 34, as shown in Figure 4.
As illustrated in F1gure 4, an apparatus 30, such as a
li~uid-crystal display unit can be électrically connected to a
printed circuit board 32 by layered strip connector~ 34. Each
of the electrical conductors 36 on the printed circuit board
30 is roughly positloned to correspond with a conductor on the
display unit 30. The electrically conductive layers of the
connector 34 permit electrical current to travel between each
of the corresponding conductors. The non-conductive layers
prevent electrical current from traveling to non-corresponding
conductors. The number of layers present in the connector
34 is typically several times the number of conductors 36 on
the printed circuit board.
The resilient character of the elastomers used in pro-
ducing the connector 34 cushions and absorbs shock and vibration
between the apparatus 30 and the circuit board 32. The smooth
compliant surface of the connec~or 34 seals the surface of the
conductors 36 after contact inhibiting contact corrosion.
A connector 40, illustrated in Figure 5, consisting of a
number of layered connector elements 42 retained in substantially
fixed relation to one another by retaining means 44 can be
advantageously used in the place of the independent strip con-
nectors 34 of Figure 4. The retaining means can be either con-
ductive or non-conductive as the particular situation might
demand~
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1056031
A connector such as connector 40 can be conveniently made
by sandwiching a layered block of elastomers 38, similar to
block 20 of Figure 2~ between two laminae 46 as shown in Figure
6. The two laminae 46 can be made of any desired materials and
can be multilayered, but to achieve greatest strength preferably
are made of single layers of an elastomer which is fully compatible
with the elastomers used to form the individual layers of block
38. The layered block of elastomers 38 and the antipodally
bordering laminae 46 are cured or vulcanized together to form a
single block 48 from which slabs 50 can be sliced.
The slab 50 consists of elongated elements 52, similar
to rods 24 and 26 of Figure 2, which are formed from the layered
elastomers 38 and are alternatingly conductive and non-conductive,
bounded by bordering elements 54 formed from the laminae 46.
One or more connectars 40, illustrated in Figure 5, can be cut,
stamped, punched or otherwise formed from slab 50~ The waste
from this process can be used to form other layered connectlng
elements generally-according to this invention. The bordering
elements 54 of slab S0 form the retaining means 44 of connector
40. The connector 40 can be viewed as a slab 50 having at least
one inside edge 56 defining an orifice through the slab.
Another connector according to this invention, having
particular utility in connecting very small, fraglle electrical
circult elements such as integrated circuit chips and the like
can be made by bo~dlng one or more slabs 22 of elongated elements
of alternately conductive and non-conductive elastomer 24 and
26 to a body 72, as shown in Figure 8. The body 72 is preferably
an elastomer fully campatible with the elastomers orming the
slabs 22. The body 72 acts as a retaining means 44 for sub-
stantially fixing the phys.ical relationship between the slabs
- 16 -
1056~31
22. The body 72 and slabs 22 are then cut in planes to which
the layers of conductive and non-conductive elastomer 24 and
26 are essentially perpendicular; thus, generating the connector
74 shown generally in Figure 9.
~he connector 74 comprises a central portion 76 ha~ing
upper and lower surfaces 78 and 80, respectively, bounded by
and joined by a periphery 82. Ftxed to the periphery is a
plurality of strips 10, each strip comprising alternate layers
` of electrically conducttve and electrically non-conductive elastomer,
each layer extending between and being cotermin s with the upper
and lower surfaces 78 and 80. The central portion 76 can be
tailored to have any shape desired, whether it ~e rectangular,
circul æ , or other convenient shape. ~he central portion 76
functions as a retaining means 44 to fix the physical.relation-
sh~ps between the two strips 10. The upper ana lower surfaces
78 and 80, while generally being parallel to each other, can
be skewed, one with respect to another, in certain circumstances.
In general, the area of the periphery 82 is less than the area
of either the upper surface 78 or lower surface 80. Further,
the linear dimension of each strip 10 transversely perpendicular
to the alternate layers 24 and 26 forming the str~p 10 is at .
least several times the greatest linear dimension of any of the
alternate layers forming the strip,
Although the inventton has been described in considerable
detail with reference to certain preferred embod~ments thereof,
it will be understood that variatiOns and modifications can be
effected withi~ the spir~t and scope of the tnvention as des-
cribed above and as defined in the attached claims.
- 17 -
