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REFERENCE TO RELATED APPLICATION
This Application is a Continuation-in-Part of U.S. Pat. application Ser.
No. 07/145,612, filed Jan. 19, 1988, to the same Applicant and entitled
Process for Producing Electric Resistors having a Wide Range of Specific
Resistance Values.
BACKGROUND OF THE INVENTION
The present invention relates to a continuous, flexible electric conductor
suitable for employment on an electric line, and capable of functioning as
an electric switch. Electric current is known to be supplied between
source and user equipment over an electric line, to which the said
elements are series-connected, and which also comprises at least one
electric switch, also series-connected to the line and which, when closed,
allows current to flow from the source to the user equipment.
For controlling the electric circuit at various points along the said line,
provision is made for a number of switches, each series-connected
electrically to the source and user equipment. In this case, the line
comprises at least two conducting wires, which must be connected, e.g.
welded, to the connecting terminals on the said switches, as well as to
the terminals on the source and user equipment.
An electric line of the aforementioned type therefore involves a
considerable number of both connections and component parts (i.e.
switches), the consequences of which are high cost and greater breakdown
potential along the line caused, for example, by loose wires or
infiltration, e.g. by water, on the switch connecting terminals.
Furthermore, changes to such a line, e.g. re-allocation of the switches,
can only be made with difficulty, which also applies to re-utilization of
the component parts of the line (conducting wires and switches).
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a continuous, flexible
electric conductor also capable of functioning as an electric switch, and
which provides for forming electric lines involving none of the
aforementioned drawbacks.
With this aim in view, according to the present invention, there is
provided a continuous, flexible electric conductor, characterised by the
fact that it comprises a first elongated electric conducting element; a
spacer element formed from insulating material and placed over the surface
of the said first conducting element, so as to shield all but given
portions of the said surface; a second tubular electric conducting element
placed over the outside of the said spacer element; a third tubular
electric conducting element placed over the outside of the said second
element; and a tubular insulating sheath placed over the outside of the
said third conducting element; the structure of the said second conducting
element comprising a supporting matrix formed from flexible, electrically
insulating material and particles of electrically-conductive material
scattered in random, substantially uniform manner inside cells on the said
matrix; said cells communicating at least partially with one another, and
being at least partially larger in size than the respective particles of
said electrically-conductive material housed inside the same.
The said structure of the said second electric conducting element is of the
type described in U.S. Pat. application No. 07/145,612 filed Jan. 19,
1988, and entitled: "Electric resistor designed for use as an electric
conducting element in an electric circuit, and relative manufacturing
process.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described, by way of example, with reference
to the accompanying drawings, in which:
FIG. 1 shows a longitudnal section of a length of the conductor according
to the present invention;
FIG. 2 shows an enlarged longitudinal section of a length of the said
conductor;
FIG. 3 shows the structure of the material with which is formed the second
electric conducting element forming part of the electric conductor
according to the present invention;
FIG. 4 shows a view in perspective of a length of the conductor according
to the present invention connected to an electrical source, a user device,
and a device for generating pressure on the conductor and so closing the
electric circuit formed by the said components and conductor;
FIGS. 5 and 6 show two structural sections, to different scales, of a
portion of the resistor according to the present invention;
The graphs in FIGS. 7 to 10 show the variation in electrical resistance of
the resistor according to the present invention, as a function of the
pressure exerted on the resistor itself;
FIG. 11 shows a schematic diagram of a test circuit arrangement for
plotting the results shown in FIGS. 7 to 10; and
FIGS. 12 to 16 show schematic diagrams of the basic stages in the process
for producing the electric resistor according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The continuous, flexible electric conductor according to the present
invention, a short length of which is shown in FIG. 1, comprises a first
elongated electric conducting element 1, and a spacer element 2 formed
from insulating material and placed over surface 3 of the said first
element, in such a manner as to shield all but given portions of the said
surface 3. In the embodiment shown in the accompanying drawings, the said
spacer element 2 substantially consists of a continuous tape wound about
the said surface 3, the said exposed portions therefore consisting of the
portions of surface 3 lying between successive turns of the said tape.
The conductor according to the present invention also comprises a second,
tubular electric conducting element 4 having its inner surface resting on
the outer surface of the said spacer element 2; a third, tubular electric
conducting element 5 having its inner surface resting on the outer surface
of the said second element 4, as shown clearly in FIG. 1; and a tubular
sheath 6 formed from insulating material and placed over the said third
conducting element 5.
The structure of the material from which the said second conducting element
4 is formed is as shown in FIG. 3, and substantially comprises a
supporting matrix 7 formed from flexible, electrically-insulating material
and particles 8 of electrically-conductive material scattered in random,
substantially uniform manner inside cells on the said matrix. The said
cells communicate, at least partially, with one another, and are, at least
partially, larger than the respective particles of electrically-conductive
material housed inside the same, so as to leave a gap 9 (FIG. 3) between
the outer surface of each particle and the surface of the respective cell.
The above material is described in detail in U.S. Pat. application No.
07/145,612 filed Jan. 19, 1988, by the present Applicant and entitled:
"Electric resistor designed for use as an electric conducting element in
an electric circuit, and relative manufacturing process", the entire
disclosure of which is incorporated herein by reference.
As stated in the above patent application, the said material is
electrically-conductive enough for it to be actually employed as an
electric conductor. Furthermore, when pressure is applied on the said
material, there is a fall in electric resistance measured perpendicular to
the pressure direction; which fall in resistance increases alongside
increasing pressure.
Such favourable performance is probably due to improved electrical
conductivity of chains of particles 8. In fact, in addition to improving
the conductivity of contacting particle chains, increasing pressure also
renders conductive any chains having gaps 9 between adjacent particles, by
bridging the said gaps 9 and so enabling adjacent pairs of otherwise
non-conductive particles to become conductive when sufficient external
pressure is applied.
To enable a clearer understanding of the process according to which the
second conducting element 4 is formed, a description will first be given
of the structure of the resistor so formed.
The structure of the resistor is as shown in FIGS. 5 and 6, which show
sections of a portion of the resistor enlarged a few hundred times.
The said resistor substantially comprises a supporting matrix 214, formed
from flexible, electrically insulating material, and particles 215 of
electrically conductive material arranged in substantially uniform manner
inside corresponding cells 230 on the said matrix 214. As in the
embodiment shown, the said particles preferably consist of granules of
electrically conductive material. As shown in the larger-scale section in
FIG. 6, at least some (e.g. 50 to 90%) of the said cells communicate with
one another, and in a number of cases, are exactly the same shape and size
as the granules contained inside. Other cells, on the other hand, are
slightly larger than the said granules, so as to form a minute gap 216
between at least part of the outer surface of the granule and the
corresponding inner surface portion of the respective cell.
The arrangement of cells 230, and therefore also of granules 215, inside
matrix 214 is entirely random. Though the advantages of the resistor
according to the present invention are obtainable even if only a few of
cells 230 communicate with one another, it is nevertheless preferable for
most of them to do so. For best results, the estimated percentage of
communicating cells is around 50-90%.
Though conducting granules 215 may be of any size, this conveniently ranges
between 10 and 250 microns. Likewise, granules 15 may be of any shape and,
in this case, are preferably irregular, as shown in FIGS. 5 and 6.
Matrix 214 may be formed from any type of electrically insulating material,
providing it is flexible enough to flex, when a given pressure is applied
on the resistor, and return to its original shape when such pressure is
released. Furthermore, the material used for the matrix must be capable of
assuming a first state, in which it is sufficiently liquid for it to be
injected into a granule structure statistically presenting each of the
said granules arranged at least partially contacting the adjacent granules
with which it defines a number of gaps; and a second state in which it is
both solid and flexible. The viscosity of the liquid material conveniently
ranges from 500 to 10,000 centipoise.
Matrix 214 may conveniently be formed from synthetic resin, preferably a
synthetic thermoplastic resin, which presents all the aforementioned
characteristics and is thus especially suitable for injection into a
granule structure of the aforementioned type.
Though the size of granules 215, which depends on the size of the resistor
being produced, is not a critical factor, the said granules are preferably
very small, ranging in size from 10 to 250 microns.
The conducting material used for the granules may be any type of metal,
e.g. iron, copper, or any type of metal alloy, or non-metal material, such
as graphite or carbon. The materials for matrix 214 and granules 215 may
thus be selected from a wide range of categories, providing they present
the characteristics already mentioned.
The material employed for matrix 214 which, as already stated, must be
flexible and insulating, is preferably, though not necessarily, so
precompressed inside matrix 214 itself as to exert sufficient pressure on
particles 215 to maintain contact between the same. It follows, therefore,
that each minute element of the said matrix 214 material is in a
sufficiently marked state of triaxial precompression as to exert on
adjacent elements, in particular particles 215, far greater stress, for
producing contact pressure between the surfaces of the said particles,
than if the said triaxial precompression were not provided for. As will be
made clearer later on, such a state of triaxial precompression is a direct
consequence of the process according to the present invention.
With the structure described and shown in FIGS. 5 and 6, the resistor
according to the present invention presents an extremely large number of
granules 215 of conducting material, which granules either contact one
another, or are separated from adjacent granules by extremely small gaps
216 which may be readily bridged when given pressure is applied on the
resistor. This results in the formation, inside the said structure, of a
number of electrical conductors, each consisting of a chain comprising an
extremely large number of granules 215, which are normally already
arranged contacting one another inside the said structure. Each of the
said chains may electrically connect end surfaces 50 and 60 on the
resistor directly, as shown by dotted line Cl in FIG. 5. Alternatively,
chains may be formed inside the resistor, as shown by dotted line C2 in
FIG. 5, in which the individual granules in the chain are partly arranged
contacting one another directly, and partly separated solely by gaps 216.
The granules in such chains may be brought into contact, as in the case of
chain Cl, by subjecting surfaces 50 and 60 on the resistor to a given
pressure sufficient to flex the material of matrix 214 so bridge the said
gaps for bringing the adjacent granules separated by the same into direct
contact.
The process according to the present invention is as follows.
The first step is to prepare a homogeneous system comprising particles,
preferably granules, of a first electrically conductive material arranged
in substantially uniform manner inside a mass of a second liquid material
which, when solidified, is both electrically insulating and flexible. The
mass of the said second liquid material is then solidified to form a
supporting matrix for the granules. According to the present invention,
throughout solidification of the said second material, a given pressure is
applied on the system for the purpose of producing triaxial precompression
of the said second material when solidified. Such pressure, which is
maintained substantially constant throughout solidification, ranges from a
few tenths of a N/mm.sup.2 to a few N/mm.sup.2.
For forming the said homogeneous system, a granule structure is first
formed, which structure statistically presents each granule arranged at
least partially contacting the adjacent granules, with which it defines a
number of gaps which are then injected with the said second liquid
material. The said second material may be liquified by simply heating it
to a given temperature. For solidifying it, cooling is usually sufficient.
In the case of synthetic resins, however, these must be solidified by
means of curing.
The process according to the present invention may comprise the following
stages.
A first stage, in which a mass of electrically conductive granules 116 is
formed, for example, inside an appropriate vessel 115 (FIG. 12). For this
purpose, the granules, after being poured into the said vessel, are
vibrated so as to enable settling. The bottom of vessel 115 is
conveniently either porous or provided with holes for letting out the air
or gas trapped between the granules.
A second stage, as shown in FIG. 13, in which the mass of granules 116 is
compacted by subjecting it to a given pressure, e.g. by means of piston
117, applied in any appropriate manner on the upper surface of mass 116.
This produces a granule structure in which, statistically, at least part
of the surface of each granule is arranged contacting surface portions of
the adjacent granules, with gaps inbetween.
As shown in FIG. 13, piston 117 is conveniently provided with a tank 118
containing the said second material in liquid form; which liquid material
may be forced, e.g. by a second piston 119, through hole 120 into a
chamber 121 defined between the upper surface of granules 116 and the
lower surface of piston 117 as shown clearly in FIG. 14. The said second
liquid material in tank 118 is a material which may be solidified and,
when it is, is both insulating and flexible. In the event the said
material is liquified by heating, appropriate heating means (not shown)
are also provided for.
A third stage (FIGS. 14 and 15) in which piston 119 moves down and piston
117 up, so as to force a given amount of the said second liquid material
inside chamber 121 (FIG. 14). Piston 117 is then brought down for
producing a given pressure inside the liquid material in chamber 121 and
so forcing it to flow into the gaps between the granules in mass 116 and
form, with the said granules, the said homogeneous system. At the same
time, any air between the granules is expelled through the porous bottom
of vessel 115. The pressure produced by piston 117, at this stage, inside
the liquid material mainly depends on the size of the granules, the
viscosity of the liquid, the height of the granule mass being impregnated,
and required impregnating time.
Penetration of the liquid material inside the gaps in granule mass 116 has
been found to have no noticeable effect on the granule arrangement
produced in the compacting stage.
A fourth stage (FIG. 15) in which the homogeneous system of granules and
liquid material produced in the foregoing stage is substantially
solidified. This may be achieved by simply allowing the system to cool and
the said second liquid material to set. At this stage, changes may be
observed in the structure of the said second material due, for example, to
curing of the same.
It has been found necessary to dose the liquid material fed into chamber
121 prior to the injection stage, in such a manner as to ensure that it is
sufficient to impregnate only a large part of granule mass 116 leaving a
nonimpregnated layer 122 (e.g. of about 25%). In like manner, the liquid
material flowing inside the gaps between the granules is subjected solely
to atmospheric pressure through the porous bottom of vessel 115. The
granules, on the other hand, (be they impregnated or not), are subjected
to the pressure exerted by piston 117, as shown in FIG. 16. The said
pressure is applied evenly over all the contact points between adjacent
granules, and is what determines the specific electrical resistance of the
resulting material. That is to say, using the same type of granules and
liquid material, an increase in the said pressure results, within certain
limits, in a reduction of the specific electrical resistance of the
resulting material. The said pressure must be maintained constant until
the liquid material has set, and must be at least equal or greater than
the compacting pressure applied in stage 2 (FIG. 13).
Though the said pressure may be selected from within a very wide range,
convenient pressure values have been found to range from a few tenths of a
N/mm.sup.2 to a few N/mm.sup.2. For resistors prepared as described in the
following examples, the following pressures were selected:
Example 1 : 1.17 N/mm.sup.2
Example 2 : 0.62 N/mm.sup.2
Example 3 : 1.56 N/mm.sup.2
Example 4 : 2.35 N/mm.sup.2
Example 5 : 1.17 N/mm.sup.2
The mass of material so formed inside vessel 115 may be cut, using standard
mechanical methods, into any shape or size for producing the electric
resistor according to the present invention.
To those skilled in the art it will be clear that changes may be made to
both the resistor and the process as described and illustrated herein
without, however, departing from the scope of the present invention.
In particular, granules 215 arranged inside matrix 214 may be replaced by
particles of electrically conductive material of any shape or size, e.g.
short fibres.
For preparing the said homogeneous system comprising particles of a first
electrically conductive material distributed inside a mass of a second
liquid material which, when solidified, is both electrically insulating
and flexible, processing stages may be adopted other than those described
with reference to FIGS. 12 to 16.
The said homogeneous system, in fact, may be obtained by mixing the said
particles mechanically with the said second liquid material, using any
appropriate means for the purpose.
According to the aforementioned variation, throughout solidification of the
said second material, the said system is forced against a porous (or
punched) septum for letting out, through the said septum, at least part of
the said second liquid material. The pressure so produced may be
maintained until the said second material solidifies, so as to produce the
said triaxial precompression in the solidified said second material.
For achieving the said precompression, the said system may be spun
throughout solidification of the said second liquid material.
When incorporated in an electric circuit, performance of the resistor
according to the present invention is as follows.
If no external pressure is applied on the resistor, and end surfaces 50 and
60 are connected electrically via appropriate conductors, electric current
may be fed through the resistor as in any type of rheophore. The density
of the current feedable through the resistor has been found to be very
high, at times in the region of ten A/cm.sup.2. When idle, the resistance
of the resistor according to the present invention may, therefore, be low
enough to produce an electrical conductor capable of handling a high
current density, as required for supplying a circuit component or device.
A number of resistance values relative to resistors produced by
appropriately selecting the characteristics of the particles and the
material of matrix 214, and the parameters of the present process, are
shown in the Examples given later on.
Total resistance of the resistor so formed has been found to be constant,
and dependent solely on the structure of the resistor, in particular, the
number and size of communicating cells 230 in matrix 214, and the number
of gaps 216 separating adjacent granules 215.
By appropriately selecting the aforementioned parameters, some of which
depend on the process described, a resistor may be produced having a given
prearranged resistance. When pressure is applied perpendicularly to
surfaces 50 and 60, the electrical resistance measured perpendicularly to
the said surfaces is reduced in direct proportion to the amount of
pressure applied. FIGS. 7 to 10 show four resistance-pressure graphs by
way of examples and relative to four different types of resistors, the
characteristics of which will be discussed later on. As shown in the said
graphs, the fall in resistance as a function of pressure is a gradual
process represented by a curve usually presenting a steep initial portion.
Even very light pressure, such as might be applied manually, has been
found to produce a considerable fall in resistance. In the case of a
resistor having the resistance-pressure characteristics shown in FIG. 10,
starting resistance was reduced to less than one percent by simply
applying a pressure of around 1 N/mm.sup.2 (about 10 kg/cm.sup.2). With a
different structure and pressures of around 2 N/mm.sup.2 (about 20
kg/cm.sup.2), starting resistance may be reduced by 1/3 (as shown in the
FIG. 7 graph).
If the pressure applied on the resistor according to the present invention
is maintained constant (or zero pressure is applied), electrical
performance of the resistor has been found to conform with both Ohm's and
Joule's law. For application purposes, it is especially important to
prevent the heat generated inside the resistor (Joule effect) from
damaging the structure. This obviously entails knowing a good deal about
the thermal performance of the material from which the supporting matrix
is formed.
Assuming the resistor according to the present invention is capable of
withstanding an average maximum temperature of 50.degree. C., under normal
heat exchange conditions with an ambient air temperature of 20.degree. C.,
the density of the current feedable through the resistor ranges from 0.2
A/cm.sup.2 (Example 4) to 11 A/cm.sup.2 (Example 5) providing no external
pressure is applied.
In the presence of external pressure, such favourable performance of the
electric resistor according to the present invention is probably due to
improved electrical conductivity of granule chains such as C1 and C2 in
FIG. 5. In fact, as pressure increases, the conductivity of
contacting-granule chains (such as C1) increases due to improved
electrical contact between adjacent granules, both on account of the
pressure with which one granule is thrust against another, and the
increased contact area between adjacent granules. In addition to this,
granule chains such as C2, in which the adjacent granules are separated by
gaps 216, also become conductive when a given external pressure is applied
for bridging the gaps between adjacent pairs of otherwise non-conductive
granules.
Total electrical conductivity of the granule chains increases gradually
alongside increasing pressure by virtue of matrix 14 being formed from
flexible material, and by virtue of the said material being precompressed
triaxially. As a result, adjacent granules separated by gaps 216 are
gradually brought together, and the contact area of the granules already
contacting one another is increased gradually as flexing of the matrix
material increases. Each specific external pressure is obviously related
to a given resistor structure and a given total conducting capacity of the
same. When external pressure is released, the resistor returns to its
initial unflexed configuration and, therefore, also its initial resistance
rating.
In the said initial unflexed configuration, the electrical performance of
the material the resistor is made of has been found to be isotropic, in
the sense that the specific resistance of the material is in no way
affected by the direction in which it is measured. If, on the other hand,
the material the resistor according to the present invention is made of is
flexed by applying external pressure in a given direction, the specific
resistance of the material has been found to vary continuously in the said
direction, depending on the amount and direction of the flexing pressure
applied.
To illustrate the electrical performance of the resistor according to the
present invention, when subjected to varying external pressure, four
resistors featuring different structural parameters will now be examined
by way of examples.
A fifth example will also be examined in which the specific resistance of
the resistor according to the present invention is sufficiently low for it
to be considered a conductor.
EXAMPLE 1
A cylindrical resistor, 12.6 mm in diameter and 7.4 mm high was prepared,
as shown in FIGS. 12 to 16, using epoxy resin (VB-BO 15) for matrix 214.
Conducting granules 215 consisted of carbon powder ranging in size from 200
to 250 microns.
On resistors with granules of this sort, the matrix insulating material
injected between the granules occupies approximately 56.8% of the total
volume of the resistor. The resistor so formed was connected to the
electric circuit in FIG. 11 in which it is indicated by number 110. The
said circuit comprises a stabilized power unit 111 (with an output
voltage, in this case, of 4.5V), a load resistor 112 (in this case, 10
ohm), and a digital voltmeter 113, connected as shown in FIG. 11. Resistor
110 was subjected to pressures ranging from 7.8 . 10.sup.-2 N/mm.sup.2 to
196 . 10.sup.-2 N/mm.sup.2.
Resistance was measured by measuring the difference in potential at the
terminals of resistor 112 using voltmeter 113, and plotted against
pressure as shown in the FIG. 7 graph. From a starting figure of 5.4 Ohm,
resistance gradually drops down to 1.78 Ohm as the said maximum pressure
is reached.
EXAMPLE 2
A cylindrical resistor, 12.6 mm in diameter and 7.2 mm high was prepared as
before using an alpha-cyanoacrylatebase resin for matrix 214 and carbon
granules ranging in size from 200 to 250 microns.
Once again, the resistor was connected to the FIG. 11 circuit, the
components of which presented the same parameters as in Example 1. The
relative resistance-pressure graph is shown in FIG. 8, which shows a
resistance drop from 16 to 5.25 Ohm between the same minimum and maximum
pressures as in Example 1.
EXAMPLE 3
A tubular resistor with an outside diameter of 12.6 mm, an inside diameter
of 3.5 mm, and 5.4 mm high was prepared as before, using epoxy resin
(VB-BO 15) for the matrix and iron granules ranging in size from 50 to 150
microns. On resistors with granules of this sort, the matrix insulating
material injected between the granules occupies approximately 55% of the
total volume of the resistor. Resistance was again measured as shown in
FIG. 11 using a 1000 Ohm load resistor 112 and 4.5 V power unit 111.
Pressure was adjusted gradually from 59 . 10.sup.-2 N/mm.sup.2 to 7.22
N/mm.sup.2 to give the graph shown in FIG. 9, which shows a resistance
drop from 1790 to 493 Ohm between minimum and maximum pressure.
EXAMPLE 4
A 2.4 mm high tubular resistor having the same section as in Example 3 was
prepared as before, using silicon resin for matrix 214 and iron granules
ranging in size from 50 to 150 microns.
Resistance was again measured on the FIG. 11 circuit, using a 100 Ohm load
resistor 112 and a 1.2 V power unit 111. Pressure was adjusted from 4.2 .
10.sup.-2 N/mm.sup.2 to 119. 10.sup.-2 N/mm.sup.2 to give the graph shown
in FIG. 10 which shows a resistance drop from 1100 to 8.1 Ohm between
minimum and maximum pressure.
EXAMPLE 5
A 3.4 mm high tubular resistor having the same section as in Example 4 was
prepared as before, using epoxy resin (VB-ST 29) for matrix 214 and tin
granules ranging in size from 50 to 200 microns.
Resistance, measured in the absence of external pressure between the two
bases of the tubular-section cylinder, was 0.08 Ohm. The specific
resistance of the resistor material, in this case, therefore works out at
0.27 Ohm.cm, which is low enough for the resistor to be considered a
conductor. Assuming heat (Joule effect) is dissipated by normal heat
exchange in air at a temperature of 20.degree. C., and the maximum
temperature withstandable by the resistor is 50.degree. C., the density of
the current feedable through this resistor is approximately 11 A/cm.sup.2.
The said first conducting element 1 conveniently consists simply of a
number of metal wires, whereas the said third electric conducting element
5 consists of a plait of metal wires defining a tubular casing.
The said spacer element 2 may be formed differently from the one described
herein, and may comprise, for example, a number of separate spacer
elements arranged contacting the outer surface 3 of conducting element 1;
or a tube of flexible material having perforations for exposing given
portions of surface 3 of conducting element 1; or even a braid formed from
insulating material.
Conducting elements 1 and 5 may also be structured differently from those
described herein.
The electric conductor according to the present invention may be connected
to an electric circuit as shown in FIG. 4, by series-connecting the first
and third electric conductors, 1 and 5, to a current source, of which FIG.
4 shows terminals 10, and to a user device 11. When connected as shown,
the conductor may also be operated as a switch, by applying given,
relatively low pressure in any manner on the outer surface of the
conductor. For this purpose, provision may be made for a grip 12 inside
which a length of the conductor is placed, and which provides for exerting
substantially radial pressure on the outer surface of the conductor, when
arms 13 on the said grip 12 are pressed together in the direction of the
conductor axis. Manual pressure applied directly on the conductor by the
user, e.g. by gripping the conductor between two fingers, is also
sufficient for the purpose.
If no pressure is applied on the outer surface of the conductor, no current
circulates in the line so formed. In fact, the said first and third
conductors, 1 and 5, connected to the current source and user device, are
insulated from each other by spacer element 2; and the portions of surface
3 of conducting element 1 left exposed by the said spacer element 2 are
separated from the inner surface of conducting element 4 by a layer of
air, thus cutting off current flow between conducting elements 1 and 4.
When, on the other hand, pressure is applied on the outer surface of the
conductor according to the present invention, e.g. using grip 12 in FIG.
4, portion 14 (FIG. 2) on which the said pressure is applied flexes
radially, substantially as shown in FIG. 2, so as to bring inner surface
15 of the said portion 14 substantially into contact with outer surface 3
of conducting element 1 left exposed by spacer element 2. Localised
electrical contact is thus established between conducting elements 1 and 4
on portion 14, thus enabling current to flow substantially radially along
conducting element 4, so as to close the FIG. 4 electric circuit inside
which current is allowed to flow. Flexed portion 14 of the conductor
according to the present invention thus functions as a switch, capable of
closing the said circuit when radial pressure is applied on the said
portion 14.
The said switch function may, of course, be performed by any short portion
along conductor 4, which thus provides, in a simple, straightforward
manner, for forming an electric line requiring a number of electric
switches. What is more, the said line may be formed with no connections
required to switch terminals or electric conductors. Switches formed
according to the present invention also provide for greater reliability,
by virtue of the contact surfaces for closing the said circuit being
airtight and fully insulated from the outside atmosphere.
When pressure is removed from the outer surface of the conductor according
to the present invention, the said second conducting element 4 returns to
its original shape, thus opening the said circuit. This is achieved by
virtue of the high degree of elasticity of the material from which the
said conducting element 4 is formed, and the characteristics of which are
described in detail in the aforementioned patent application. A further
characteristic of the said material is that its electrical conductivity,
and therefore also the amount of current flowing along the said line,
increases alongside increasing pressure on the material, which favourable
property may be employed to advantage in the construction of the said
line. Furthermore, by replacing the said conducting element 1 with a
calibrated resistor and selectively flexing a number of conductor
portions, one at a time, it is possible to determine which of the said
portions has been flexed, by measuring total resistance along the line. In
other words, the system functions in the same way as a rheostat, the wiper
of which is set to various flexure points on element 4.
To those skilled in the art it will be clear that changes may be made to
the electric conductor as described and illustrated herein without,
however, departing from the scope of the present invention.
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