US2250781A

US2250781A - Microphone

Info

Publication number: US2250781A
Application number: US209507A
Authority: US
Inventors: Sutton George William
Original assignee: Siemens Brothers and Co Ltd
Current assignee: Siemens Brothers and Co Ltd
Priority date: 1934-11-27
Filing date: 1938-05-23
Publication date: 1941-07-29
Anticipated expiration: 1958-07-29

Description

3 Sheets-Sheet l G. W. SUTTON MICROPHONE Original Filed Nov. 25, 1935 July 29, 1941.

IINVENTOR. GEORGE WILLIAM SUTTON ATTORNEY.

FIGJ

Jul 29, i941.'

G. w. SUTTON MICROPHONE 3 Sheets-Sheet 2 Original Filed Nov. 25, 1935 FIG. 3

Fre.4

July 29, 1941.

G; w. SUTTON MICROPHONE Original Filed Nov. 25, 1935 3 Sheets-Sheet 5 x Ammim FIG. 5

INVENTOR. GEORGE WILLIAM SUTTON I ATTORNE Patented July 29, 1941 signor to Siemens Brothers & Company Limited, London, England Original application November'25, 1935; Serial No. 51,501, new Patent No. 2,179,733, dated November 14, 1939. Divided and this :application May 23, 1938, Serial No. 209,507. Britain November 27, 1934 .InGreat 13 Claims. (91. 179-435) The present invention relates to microphones or transmitters such as are used in telephone, public address and sound recording systems and more particularly to improvements in the form and construction of microphones or transmitters of the difierential type in which carbon or other conducting granules are used.

This application is a division of my co-pending application Serial No. 51,501, filed November 25, 1935, and the claims herein are directed to the paths to be increased as the resistance of the other path is decreased. In the usual differential transmitter a moving electrode which is coupled to the diaphragm is situated between two fixed electrodes. It is also feasible to have three, fixed electrodes, the pressure of the granules being varied by an element coupled to the diaphragm.

According to one feature of the invention, a difierential microphone or transmitter is constructed so that with a proper filling of granules in the granule chamber a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter. This construction helps among other things to ensure the two halves of the transmission system corresponding to the two current paths referred to in the preceding paragraph are maintained substantially in balance under all working conditions. With a difierential microphone or transmitter it is important that a sufiicient degree of balance should be maintained, as a lack of balance results not only in a decreased eificiency' of conversion from sound waves to alternating currents but also in a; diminution of that freedom from the production of even harmonies and from trouble due to a noisy source of microphone current which should result from push-pull operation. The terms microphone and transmitter are used in a wide sense, and are intended to cover a transmitter element with or without parts not concerned with the invention such as a holding or mouthpiece.

In applying the invention to a difierential microphone or transmitter of the kind comprising two fixedelectrodes and a moving electrode, it is preferably arranged that the maintenance of a substantial head of granules on the active face of each fixed electrode at all positions of the microphone r transmitter is due tothe use of a construction whereinxthe activexfaceoteach fixed .electrode :stands away from theend of the granulejchamberon whichthe electrode is supported and wherein aspace for the accommodation of granules exists around the body. of; the

electrode; :and/or around. axmember. supporting the electrode .:and projecting into. the granule chamber. A construction may be used wherein a diaphragm to :which the. moving electrode is coupled has a .central portion consisting of.:. a cone .:ofthin-'smaterial wherein .;the apex .:of.;this' cone is..-attached :to. the :moving electrode-bya rod or pin which passeszthrough a hole in a fixed electrode lying between; the .diaphragmand. the moving electrode, andrwhereinthe .apexzis. situ ated within a depression in awallof the granule chamber, this depression having its counterpart inside the granule chamberrin a protuberance on which the said fixed electrode is mounted. This construction has the advantage that it allows the rod or pinto be short and rigidand yet provides for the immersion in the granules of the fixed electrode concerned.

In certain contemplatedembodimentsof the.

invention comprising microphones or transmitters of the kind having two fixed electrodes and a moving electrode, the overall internal dimensions of the granule chamber in directions transverse to its length are considerably greater than would be'necessary just to accommodate the fixed and moving electrodes, and according to a subordinate feature of the invention in cases where such a construction is employed aninsulatingbarrier is provided round the moving electrode to prevent or minimize the fiow of current through the granules in paths not including this electrode and in paths the resistance of which is not effectively variedby movement of this electrode. The insulating barrier may be a partial one allowing granules to pass freely from one section of the granule chamber to the other, or it may be a complete barrier, in which case care must be taken to ensure that inspite of the presence of the barrier a substantial head of granules acts on the active faces of the moving electrode atall positions of the microphone or transmitter.

The invention is applicable to a difierential microphone or transmitter of the kind in which a modulating member attached to a diaphragm and situated between two fixed electrodes is not itself an electrode but acts to vary the resistance of the current paths between the two fixed electrodes mentioned and a third fixed electrode. In a contemplated embodiment of the invention comprising a microphone or transmitter of this kind, two of the fixed electrodes project from opposite ends of the granule chamber, and the active face of the third fixed electrode is in the form of a complete surface of revolution which encompasses the other two electrodes and the modulating member.

A microphone or transmitter according to the invention may be used as an ordinary subscribers or operators transmitter in a public or private telephone system, and, owing to the sensitivity resulting from the use of the differential principle, it may also, for example, be used in cases where it is desired to operate a loudspeaking receiver or receivers without the aid of an amplifier or where it is desired to render it unnecessary for the source of sound to be situated close to the transmitter.

A number of specific embodiments of the invention will now be described with reference to the accompanying drawings. The invention is not limited to-these embodiments, which are described by. way of example only. Fig. 1 of the drawings shows .a cross-sectional view of a capsule or inset transmitter according to the invention, and Figs. 2, 3, 4 and show crosssectional views of other capsule or inset transmitters according to the invention. With the exception of Figs. 1 and 5, each figure shows only part of the transmitter concerned. For convenience inlaying out the drawings, the transmitters in Figs. 2-4 are shown lying on their backs, although this would, of course, not be the usual position when in use.

Referring firstly to Fig. 1, the capsule transmitter shown has a moving electrode I situated between two

fixed electrodes

2 and 3. It is, of course, intended that the two microphone elements formed by the variable resistance path between the moving electrode and one fixed electrode, and the variable resistance path between the moving electrode and the other fixed electrode, shall be connected in the usual manner to give a push-pull effect.

The transmitter has a thin aluminum alloy diaphragm consisting mainly of a central portion 4 of conical form, this portion being surrounded by a flat annular portion 5 to which it is connected by a further portion 6 having the form of a frustum of a cone. The periphery of the diaphragm is clamped to the main casing I of the transmitter by a spun brass ring 8. The outer face of the diaphragm is stove enameled to minimize the possibility of corrosion. The casing I is of brass, and a brass guard plate 9 which fits tightly on the ring 3 and is carried thereby serves to protect the comparatively fragile diaphragm from damage. The guard plate has apertures It to allow sound waves to reach the diaphragm.

Attached to the apex of the center conical portion 4 of the diaphragm is one end of a light aluminum alloy rod or pin II, the other end of this rod being attached to the moving electrode I by means of a small aluminum alloy nut I3. Although the rod is shown in cross-section, the usual shading has been omitted in order to avoid confusion of the drawing. The parts of the transmitter are designed in such manner as to enable the rod II to be as short and rigid as is possible compatible with a permissible weight. The rod I I, the diaphragm, and the casing I serve to provide the necessary electrical connection to the moving electrode I. The moving electrode is a stiff disc of carbon having a hole drilled in its centre, the requisite rigidity compatible with a permissible weight being obtained by making the disc thick near the centre and thin at the edge. The diameter of the disc is about one fifth that of the free portion of the diaphragm, and the disc vibrates substantially as a whole at the frequencies which are important for the transmission of articulate speech. The shape of the disc tends to minimize the flow of current through the granules in paths the resistance of which is not effectively varied by movement of the disc.

In the quiescent condition, the moving electrode I is equidistant from the

fixed electrodes

2 and 3, which are carbon electrodes of annular form. It is always totally immersed in the granules, which are of carbon. The granules are not shown in the drawings, but a normal filling of granules occupies a sufficiently large portion of the available volume of the granule chamber to ensure that the active faces of all three electrodes remain totally immersed in granules at all positions of the transmitter. The surfaces of the fixed electrodes which face the moving electrode (i. e. the active faces) are shaped in such a manner that the opposing surfaces of the fixed and moving electrodes are substantially parallel. The rod I I passes through the hole in the centre of the front fixed electrode 2, a felt washer I4 surrounding the rod to prevent leakage of granules from the granule chamber. The felt washer is held in position by a further washer I5 and a sprung ring IS. The further washer I5 is an insulating washer made of a synthetic resin product. The hole in the center of the back fixed electrode 3 serves to provide the passage through which the granules are inserted into the granule chamber on the completion of the assembly of the transmitter, and also serves to make the mechanical impedance to displacement of the moving electrode more nearly the same for similar displacements in either direction and to maintain the symmetry of the two current paths through the transmitter. Since the portion of the rod I I lying within the hole in fixed electrode 2 is appreciably smaller in diameter than the hole, both the holes in the fixed electrodes serve to provide room for a small quantity of granules and thus assist to some extent in that immersion of the fixed electrodes which is a feature of the transmitter The granule chamber is carried by the casing 'I, the cylindrical brass wall ll of the chamber being insulated from the casing I by insulating material I3. In manufacturing the transmitter, the material I3 is pressed, moulded, or squirted into the correct form and position. The material is locked in position in the casing I by rivet-shaped portions such as I9, and three projecting portions 20 spaced apart with respect to the axis of the capsule lock the granule chamber in position. The material may conveniently be a material such as ebonite or a cellulose acetate compound or a synthetic resin product. The front wall 2| of the granule chamber is of brass and is fixed to the cylindrical wall [1 by the spun over edge 22 of the latter. The front fixed electrode 2 is mounted directly on the front wall so that the cylindrical wall I! serves as one terminal of the transmitter in the way that the casing 1 serves as another terminal. The back fixed electrode 3 is carried by a brass member 23 which in turn is carried by a member 24 composed of insulating material. The member 24 forms the rear wall of the granule chamber, and is fixed to the cylindrical wall I! by the spun over edge 25 of the latter. The member is secured against rotation by a punched out portion 34 of the wall H which engages a corresponding-slot in;- the member. The brass member 23 has a hollow stub 26, and the spun over edge 27 of this stub serves to fix the member 23 securely to the member 24 and also forms the third terminal of the transmitter. A screw 28 closes the passage through which the granules are inserted into the granule chamber on the completion of the-assembly of the capsule. The fixed

electrodes

2 and 3 are situated in cupshaped receptacles formed in the members 2| and 23, the spun over sides of the cups holding the electrodes firmly in position.

Air passages

29 and 30 pass through the members Hand 24, and a felt washer l2 pervious to air serves to prevent leakage of granules via these passages. The function of the passages is to ensure that the air pressure behind the diaphragm and in the granule chamber remains substantially the same as that of the outsideair.

It will be seen from the drawings that both the internal diameter and internal length of the granule chamber are greater than would be necessary just to accommodate the fixed and moving electrodes with their active faces at the appropriate distance apart (e. g. the diameter is about one and three-quarters times that of the electrodes) and that the two fixed electrodes are arranged to protrude into the granule chamber, the effect being to immerse these electrodes in the granules. allows the rod H to be short and rigid'and yet provides for the immersion of the front electrode 2 anda sufiicient air-space behind the diaphragm. The immersion of the electrodes operates to the end that a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter.

The interior walls of the granule chamber (including the surfaces of pin H and nut I3) and the exposed surfaces of the fixed electrodes other than the active faces are covered with a layer of insulating varnish or enamel.

The transmitter is of a size suitable for insertion in the transmitter container of a handmicrotelephone or in a holder of similar dimensions. The member 33 is a form of spring ring and its purpose is to force the transmitter against the mouthpiece part of a holder into which it is inserted. The punched out portions or projections 35 of the casing I serve to secure the spring-ring. The projections are four in number, three (including the one shown at the top in the figure) being spaced 120 apart with respect to the axis of the transmitter and one (shown at the bottom in the figure) being midway between two of the others. The three equally-spaced projections are spaced from the flange of the case while the fourth projection abuts it. The edge of'the central hole of the spring ring 33 is'cut away in three places corresponding to the three equally spaced projections, but each of the cuts extends for a considerable distance round the periphery of the hole. In assembling the ring in position on the casing I, it is pressed against the flange of the casing with the cuts coinciding with the four projections (the cuts are sufficiently long to enable the odd projection to enter the same cut as one of the adjacent equally spaced projections) and is then rotated until the end of one of the cuts abuts the odd projection. The spring ring has a series of holes 36 cut in it to decrease its stiffness. In so far as the terminals of the transmitter are concerned the ring where provided is looked upon as forming part of the casing I.

.It will .be realized-that the three terminals The form of the member 2| is such that it formed by the surfaces 3| and 32 of members 33 and I1 and the spun over edge 21 of stub 26 are of such a form that contact between them and-contactsprings in a casing adapted to receive-the transmitter is unaffected by rotation of the transmitter.

The passage through which the granules are inserted may alternatively'be closed by a cap permanently fixed in position by a spinning opera tion. It should be noticed that where this is done a capsule transmitter is obtained which cannot be tampered with by undoing screws or screwed parts.

In describing the embodiments shown in the remaining figures, only points in which the construction differs from that of a previously described embodiment will be dealt with in detail. For the sake of simplicity, the granular filling of the granule chamber is not shown in any of the figures.

Referring now to Fig. 2, this shows a capsule transmitter the construction of which only differs materially from that of the capsule transmitter shown in. Fig. l in that an insulating barrier is provided around the moving electrode. The barrier is formed by an annular portion of a disc or membrane 3? of very thin and flexible insulating material such as-Swiss silk.- The disc or membrane 3? has a small hole at its centre, and is threaded on the screwed portion of the rod or pin I! which couples the moving electrode to the diaphragm. The moving electrode is composed of two separate carbon discs 52 and 43 between which the central portion of the disc 3'! is clamped. The periphery of the disc 3iis clamped between two

brass members

33 and 39 which together form the cylindrical wall of the granule chamber and correspond to member I! in the capsule transmitter of Fig. 1. As shownin the figure, member 39 is held in position. by a spun over edge 43 of member 33, but it could equally well be arranged to screw into member 38. Similarly, of course, member 24 could be arranged to screw into member 39 instead of being held by a spun over edge of the latter. The unclamped annular portion of the disc 37 is slack and not taut, and this, in conjunction. with the thinness and flexibility of the material of which the disc is composed, enables the pressure due to a head of granules on one side to be readily transmitted to granules on the other side. By suitably filling the two halves of the granule chamber with granules it can thereforebe arranged that a substantial head of granules acts on both faces of the moving electrode at all positions of the transmitter. The barri r formed by the disc serves to preventthe flow of current through the granules in paths not including the moving electrode to minimize the fiow of current in paths the resistance of which is not eifectively varied by movement of this electrode.

In assembling the transmitter, the correct quantity of granules for the half of the granule chamber through which rod 5 I passes is placed in said half before the parts d2, 31, 33, and 39 are fitted and fixed in position.

In the capsule transmitter Fig. 2 justdescribed, the disc 31 may take the form of a comparatively ctiif annular washer which is of insulating material and which is supported only by being clamped at its outer edge between

members

38 and 39. The annular washer may conveniently be made of a synthetic resin product. The moving electrode in this case is in one piece as in the capsule transmitter of Fig. 1", and the central hole 'of'the annular washer is appreciably larger than. the electrode so that the annular gap around the moving electrode between the electrode and the washer is sufficiently wide to allow the free passage of granules. The annular gap is however not wide enough seriously to impair the effect of the barrier formed by the washer in so far as confining current flow to desired paths is concerned.

Referring now to Fig. 3, this shows a capsule transmitter in which the granule chamber is internally equivalent to that of the capsule transmitter described in the preceding paragraph but in which the partial barrier around the moving electrode is formed by a fin 44 of insulating material which is in one piece with the material l8 which insulates the cylindrical brass wall I! of the granule chamber from the casing l. The fin is linked with the main body of insulating material via circular holes M in the wall of the granule chamber. In manufacturing the transmitter, the insulating material is pressed, moulded or squirted into the correct form and position.

Referring now to Fig. i, this shows a capsule transmitter having a granule chamber in which a complet insulating barrier around the moving electrode is formed by an auxiliary diaphragm 55 which is fairly stiff and may for example be of mica or of metal covered with a layer of insulating varnish or enamel. The diaphragm 55 is adapted to play an important part in controlling the movement of the vibrating system of the transmitter, and makes it possible for the main diaphragm to be flexibly mounted at its edge (1. e. to be a floating diaphragm). The diaphragm 55 has a small hole at its centre, and is threaded on the screwed portion of the rod or pin H. The moving electrode is composed of two separate carbon members 51 and 58 between which the central portion of the diaphragm 55 is clamped. Each of the two carbon members consists of a ring of carbon in which a channel 6| has been out in order to make the vibrating system as light as possible. The periphery of the diaphragm is clamped between the two

brass members

38 and 39 which together form the cylindrical wall of the granule chamber. The member 24 of insulating material is clamped to member 39 by a clamping ring 55. In the case of the figure now being referred to, it has been deemed advisable to indicate the various layers of insulating varnish or enamel inside the granule chamber (such as 59, 60, 82). It will beseen that the two annular active faces of the composite moving electrode stand away from the corresponding faces of the diaphragm 55 so that spaces for the accommodation of granules exist around the body of the electrode. This construction ensures that an adequate head of granules acts on the said active faces at all positions of the transmitter. The hole 63 in the casing 1 serves to maintain equal air pressure on both sides of the main diaphragm. The main diaphragm 64 is of Referring now to Fig. 5, this shows a capsule transmitter in which all three electrodes are fixed electrodes, the resistance of the current paths between the two

similar electrodes

2 and 3 and the other electrode 56 being varied during the operation of the transmitter by a modulating member 61 attached to the diaphragm by an aluminum alloy rod or pin II. In effect, the modulating member 61 and electrode 66 together perform the functions performed by the moving electrode in the previously described embodiments. The modulating member 61 is of insulating material, and is held in position on rod II by a small aluminum alloy nut 13. Both the member and all three electrodes are solids of revolution about the axis of the capsule. Both faces of the aluminum alloy diaphragm are stove enameled to minimize the possibility of corrosion. The enamel also serves to insulate the diaphragm from: the brass casing l which in this embodiment is electrically connected to electrode 2. It will be seen that electrode 66 is a composite one consisting of two

parts

68 and 69. The active faces 10 and H of

electrodes

2 and 3 are parallel to the corresponding active faces 12 and 13 of electrode 65. The exposed surfaces of the electrodes other than the active faces, and the interior walls of the granule chamber where of metal (including the surfaces of pin H and nut !3,) are covered with a layer of insulating varnish or enamel 14. A normal filling of granules occupies a sufficiently large portion of the available volume of the granule chamber to ensure that a substantial head of granules acts on the active face or faces of each of the three electrodes at all positions of the transmitter. The felt washers l2 and Hi correspond to the washers similarly designated in Fig. 1 and serve to prevent leakage of granules from the granule chamber. The cylindrical brass wall 15 of the granule chamber is carried by the casing l but is insulated therefrom by insulating material l8. In manufacturing the transmitter, the material H3 is pressed, moulded, or squirted into the correct form and position. The front wall 16 of the granule chamber is of brass and is fixed to the casing i by a spun over edge 71 of the latter. The front electrode 2 is mounted directly on the wall it. The back electrode 3 is carried by a brass member 23 which in turn is carried by a member 24, composed of insulating material. The member 24 and the

parts

68 and 69 of electrode 65 are clamped in position in the brass member "15 by a metal clamping ring 55. The passage through which the granules are inserted is closed by a metal cap 8|.

The terminals for

electrodes

2, 3, and G8 are formed by the surfaces iii, 19, and 3B, respectively, and are hence of such a form that contact between them and contact springs in a casing adapted to receive the transmitter is unaffected by rotation of the transmitter.

What is claimed is:

1. In a differential transmitter, a microphone chamber having a cylindrical wall, a cylindrical casing surrounding said microphone wall for supporting the same, insulating means moulded between said microphone wall and said casing, three electrodes supported within said microphone chamber, said microphone wall serving as a conducting element to a terminal for one of the electrodes, said casing serving as a terminal for the second electrode, and an end wall for said microphone chamber serving as a terminal for the third electrode, each of saidterminals being circular in form so that, with said transmitter mounted in a holder having contactmaking members, rotation of the transmitter will not afiect the contact between the terminals and contact members.

2. In a differential transmitter adapted to be mounted in a holder having contact-making members therein, a microphone chamber having three electrodes supported therein, a casing for supporting said microphone chamber, insulating material moulded between said chamber and said casing permanently securing them together, an end wall for said microphone chamber insulated therefrom; said casing, said microphone chamber, and said end wall serving as terminals for the electrodes within said microphone chamber; each of said terminals being circular and making contact with the contact making member of said holder so that contact between the terminals and the contact making member is unaffected by rotation of the transmitter within the holder.

3. In a difierential transmitter, a main casing of moulded insulating material, a metallic microphone chamber having a cylindrical wall, a cylindrical metal supporting casing surrounding said microphone wall, said main casing being moulded between said microphone wall and said supporting casing to secure the same together, electrodes supported within said microphone chamber, said supporting casing and said microphone wall serving as terminals for said electrodes.

4. In a transmitter, a microphone chamber having a cylindrical wall, a casing for supporting said microphone chamber having a portion surrounding said chamber wall, an insulating separator moulded between said chamber wall and said casing, and perforations in said casing and said microphone wall cooperating with the moulded insulation to lock said elements together in their proper relationship.

5. In a transmitter, a microphone chamber having a cylindrical wall, a supporting casing for said microphone having a portion surrounding said microphone wall, an insulating member positioned between the casing and the microphone wall, said member having projections engaging openings in said chamber wall and the casing to lock the casing and chamber in proper position and prevent relative rotation therebetween.

6. In a differential transmitter, a microphone chamber having resistance material therein, three fixed electrodes supported within said chamber and insulated from one another, a diaphragm, and a modulating member of insulating material within said chamber positioned between said electrodes, said modulating member coupled to said diaphragm and actuated thereby to vary the resistance of the current path through said resistance material and between said electrodes.

7. In a difierential transmitter, a microphone chamber having resistance material therein, three fixed electrodes supported within said chamber, two of said electrodes fixed to opposite end walls of said chamber and one of said electrodes fixed to the side wall of said chamber, a diaphragm, a modulating member of insulating material in said chamber positioned between said first two electrodes, a coupling rod connecting said modulating member with said diaphragm,

said modulating member actuated by said diaphragm to vary the resistance of the current path through said resistance material between said side wall electrode and said end wall electrodes, and means for electrically insulating all three of said electrodes from each other.

8. In a differential transmitter as claimed in claim '7, a casing for supporting said microphone chamber electrically insulated from said chamber, said casing serving as a terminal for one of the end wall electrodes, said microphone chamber serving as a terminal for the side wall electrode, and a terminal member secured to the other of said end wall electrodes.

9. In a difierential transmitter, a cylindrical microphone chamber, three fixed electrodes supported within said chamber, two of said electrodes supported on opposite end walls of said chamber and the third electrode supported around the inside of the cylindrical chamber wall, said chamber wall serving as a terminal connection for said third electrode, a casing having a portion surrounding said microphone chamber but insulated therefrom serving as a terminal for an end Wall electrode, a terminal member for the other end wall electrode secured to the said end wall, a modulating member positioned between said end wall electrodes, a diaphragm positioned outside of said chamber, and a coupling rod connecting said diaphragm and said modulating member extending through one of said end wall electrodes.

10. In a differential transmitter, three concentric metallic elements, insulating means moulded between said elements to form an integral granule chamber, three stationary electrodes, each of said electrodes conductively associated with a diiferent one of said elements, a diaphragm, and means controlled by said diaphragm for varying the pressure of the granules between two of said electrodes and the third electrode.

11. A sound translating granule chamber comprising three concentric metallic elements, insulating means electrically separating said elements, three stationary electrodes mounted within said chamber, and non-conductive means in said chamber for varying the pressure of the granules, each of said electrodes electrically associated with a difierent element and each element forming a circular contacting surface for connecting its associated electrode in an electrical circuit.

12. In a differential transmitter, a chamber containing granular resistance material, three stationary electrodes mounted within said chamber, a granular contacting surface for each of said electrodes parallel to the surface of another electrode, a granular pressure varying means mounted within said chamber actuated transversely to the direction of current flow between said parallel surfaces of said electrodes.

13. In a translating device, a chamber containing granular resistance material, three fixed electrodes mounted within said chamber spaced apart from each other, means in said chamber for varying the pressure of the granules to thereby vary the resistance in current paths between said electrodes, and means for actuating said pressure varying means in directions transverse to the current paths between said electrodes.

GEORGE WILLIAM SUTTON.