US2960579A

US2960579A - Telephone transmitters

Info

Publication number: US2960579A
Application number: US692806A
Authority: US
Inventors: Beadle Anthony Crisp
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1956-11-09
Filing date: 1957-10-28
Publication date: 1960-11-15
Anticipated expiration: 1977-11-15
Also published as: FR1185908A; BE562223A; GB803284A; NL222241A

Description

1960 A. c. BEADLE TELEPHONE TRANSMITTERS 7 Filed 001;. 28, 1957 Inventor A. C. Beadle Atlorne y TELEPHONE TRANSMITTERS Anthony Crisp Beadle, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 28, 1957, Ser. No. 692,806

Claims priority, application Great Britain Nov. 9, 1956 8 Claims. (Cl. 179-122) This invention relates to telephone transmitters of the carbon granules type.

According to the invention, there is provided a telephone transmitter comprising a casing, a carbon granules chamber within said casing, and a diaphragm supported directly or indirectly upon the said casing but free to vibrate axially, the said diaphragm comprising a central electrode portion co-operating with the said carbon chamber so as to act upon a granular mass of carbon contained therein, and in which the said carbon chamber contains two concentric fixed electrodes making contact with the said granular mass, the arrangement of the said fixed electrodes and the volume of the granular mass provided being such that a substantially constant resistance between the said fixed electrodes is maintained in all positions of the said transmitter, subject only to variations arising from acoustically-generated axial excursions of the said diaphragm acting upon the said granular mass.

The principal object of the invention is to provide a high resistance carbon transmitter of the series type, that is to say, one in which the talking resistance is developed in a series path between two fixed electrodes in the carbon chamber, and capable of beingvaried by the excursions of the diaphragm. This path may, and in general does, include a conductive surface which may be a portion of the diaphragm or which'maybe a separate electrode driven by the diaphragm. The -resistance thereby developed is approximately four times that of the conventional transmitter utilising the single path between a back electrode and the front (diaphragm) electrode. Thus the talking resistance of the transmitter when new may be of the order 300400 ohms as opposed to the value of around 80 ohms for the more usual type.

The particular choice of transmitter resistance is mainly determined by the operating conditions of the DC. feeding bridge in the telephone exchange, it being a necessary condition that the feeding current shall not fall below the minimum required for operation of the supervisory or signalling relay when the exchange loop is closed. Since the electrical output of a carbon transmitter is approximately proportional to the DC. power fed to it, it follows that exchange systems with low feeding currents will require high resistance transmitters whereas systems which need higher feeding currents must use relatively low resistance transmitters.

Series-type transmitters hitherto have consisted, in the main, of an asymmetric arrangement of two or more side-by-side carbon chambers bridged by a fiat conductive diaphragm (electrode), frequently of carbon, and having the following limitations:

(i) Poor positional stability of resistance due to the use of a fiat front electrode;

(ii) The side-by-side arrangement of the fixed back electrodes gives axial asymmetry so that the transmitter capsule must be prevented from rotation;

(iii) The use of moulded carbon electrodes is expen- 2,960,579 Patented Nov. 15, 1960 sive. Moreover, the use of felt inside the capsule, as in some of theproposed designs, is objectionable.

The presentv invention seeks to avoid these limitations by novel features of construction, and the invention will now be described with reference to the single figure of the accompanying drawing, illustrating a preferred cmbodiment.

The main frame of the transmitter comprises two pressings 1 and 3 rigidly connected by a chamber ring 2, the latter being manufactured of die cast metal, such as aluminum alloy insulated'on its inner surface, or moulded from plastic material. The fastening together may be achieved by punching or spinning over of lugs on the chamber ring (as shown in the drawing) or by screws or rivets passing into or through the chamber ring. The same fastenings secure one or more fabric washers 4 between the partitions 3 and the chamber ring. These washers perform the dual function of closing the top end of the carbon chamber and providing a damping resistance across the holes 16. The acoustic mass of these holes together with the volumes on the two sides of the partition between the diaphragm 7 and back cover forms the acoustic equaliser for the transmitter, in known manner. The diaphragm 7 is protected and clamped at its outer edge by a front cover 5, which also secures the two pressings 1. and 3 at their outer edges.

The equalising and clamping arrangement just described may be achieved alternatively by a damping arrangement, in which the diaphragm is supported on tissue washers, having different damping effects at difierent frequencies. This is an indirect method of damping.

An important feature of the transmitter lies in the shape and construction of the carbon chamber, which contains two fixed electrodes and a moving electrode 8, fixed to the diaphragm 7 so that it moves axially with the diaphragm in response to the applied sound pressure. The two fixed electrodes consist of a central electrode 9 and a concentric ring electrode 10 separated by an insulating bushing 11. The bushing 11 protrudes well into the carbon chamber and supports the central electrode 9 which is shaped like a round-headed bolt, so that the head of the electrode rests in a cylindrical hollow in the bushing 11. The ring electrode 10 is stepped, as shown, the head of the bushing resting on its upper step, while the lower step is secured between the chamber ring and the back of the frame between an insulating disc 12 and an insulating washer 15. Contact is made to the ring electrode 10 by the terminal 13, insulated from the framework of pressing 1 underneath by washer 17, while the terminal end'of the central electrode is similarly insulated from the pressing l by washer 14. Additional insulation within the carbon chamber may be applied, as, for example, by enamel, to the cylindrical sides of both the front electrodes (where shown at 8) and the ring electrode (where shown at 10).

The carbon chamber thus formed is substantially filled with granular carbon, but not packed tightly, to allow for mobility of the carbon under all conditions of temperature and usage.

With this construction, the insulating bushing 11 partially obstructs the direct path for current between the electrodes 9 and 1t), and under normal operating conditions, the main current path through the carbon mass traverses the carbon twice, from one fixed electrode, to the moving electrode and back to the second electrode, and is thus subject to the modulating eifect of the acoustic excursions of the diaphragm. The shape of the insulating barrier 11 and the stepping back of the ring electrode relative to the central electrode ensure that there is no low resistance track through the carbon granules, directly between the two fixed electrodes, which might cause a loss of efiiciency. Small radial grooves (not shown) may be cut in the insulating barrier to increase the mobility of the granular carbon.

A further feature of the carbon chamber construction is the relative positioning of the electrodes and the bounding surfaces of the chamber to give a substantially constant resistance to the transmitter regardless of axial or diametric rotation. The former is achieved by axial symmetry as is normal in two electrode transmitters, but which is believed to be novel for series type transmitters. The latter is achieved by positioning the active current paths efiectively near the centre of the carbon chamber so that a useful working head of carbon is maintained regardless of the plane of the transmitter; it will be noted that in the face up position the working head of carbon on the central electrode is small but on the outer electrode large, whereas in the face down position the reverse is true so that the effect of the two paths in series is to maintain substantially constant re sistance.

Notwithstanding the above application of this trans mitter for use as a high resistance transmitter, satisfac tory operation may also be obtained by connecting the two current paths in parallel and adding a frame terminal with a conducting path to the moving electrode. Under these conditions the resistance of the transmitter would be reduced to one quarter and it would operate satisfactorily at higher feed currents.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. A telephone transmitter comprising a casing, a granular carbon chamber Within said casing, a pair of fixed concentric electrodes within said chamber, an insulating member disposed intermediate said fixed electrodes and partially shielding the inner electrode of said pair from the outer electrode, a diaphragm mounted on said casing and having a central electrode portion coopcrating with said inner electrode, said central electrode portion and a portion of said insulating member defining a relatively narrow aperture wherein a substantially constant resistance between said fixed electrodes is maintained in all positions of said transmitter.

2. A telephone transmitter as claimed in claim 1, wherein said insulating member comprises a hollow cylinder having a truncated cone-shaped portion at one end, the base of said cone-shaped portion having a wider diameter than the balance of said cylinder, a bore in the upper part of said cone portion having a diameter greater than the hollow in said cylinder.

3. A telephone transmitter as claimed in claim 2, wherein one of said fixed electrodes comprises a cylinder having a dome-shaped portion at one end, the widest diameter of said dome-shaped portion being greater than the diameter of the balance of said fixed electrode, said fixed electrode positioned within the hollow of said insulating member with its dome-shaped portion within said bore.

4. A telephone transmitter as claimed in claim 3, wherein the other of said fixed electrodes comprises an inverted cup-shaped element having an outwardly extending flange portion and a central aperture, the diameter of said aperture being substantially equal to the outside diameter of said hollow cylinder, said insulating member extending through said aperture, the bottom side f the base of said cone-shaped portion of said insulating member resting on the bottom of said inverted cup-shaped element.

5. A transmitter as claimed in claim 1, wherein the inner electrode of said pair of electrodes has its surface closer to said central electrode of said diaphragm substantially hemispherical in shape.

6. A transmitter as claimed in claim 1 and in which the said carbon chamber comprises a pair of frame members mounted within said casing, a rigid cylindrical ring supported between said frame members, one of said frame members forming a backing plate and the other providing a seating for the said diaphragm, said insulating member seated centrally of the said chamber on said backing plate so as to protrude into the said chamber, said inner electrode mounted centrally of said insulating member giving access at its inner end to the carbon mass in the said carbon chamber and at its outer end to a connecting terminal, said outer electrode mounted concentrically to said insulating member provided with a connecting terminal, said chamber being closed by said diaphragm and filled to a predetermined extent with said carbon.

7. A transmitter as claimed in claim 1, wherein the inner of the said two fixed electrodes is annular and projects into the said carbon chamber.

8.A transmitter as claimed in claim 1, wherein the central electrode portion of said diaphragm comprises a cylindrical portion concentric with the said fixed electrodes, the working surface within the said electrode portion being re-entrant and domed outwards from the said carbon chamber.

References Cited in the file of this patent FOREIGN PATENTS 153,888 Australia -i Oct. 30, 1953