Patent GB-366955-A (GB)
Title: Improvements relating to telephone transmitters
IPC: H04R21/02
Filed: 1930-11-08
Abstract: 366,955. Microphones. SIEMENS BROS. & CO., Ltd., Caxton House, Tothill Street, and GOODWIN, W. E., 72, Glyndon Road, Plumstead, both in London. Nov. 8, 1930, Nos. 33612/30 and 22691/31. [Class 40 (iv).] In order to lessen the possibility of frying, a carbon granule microphone for the conversion of sound waves into electrical current variations is provided with a part or parts in contact with the granules, which part or parts on the granules becoming heated suffer a change of shape in order to prevent a reduction of granuleelectrode pressure. Fig. 1 shows a construction of microphone of the type described in Specification 308,630, [Class 40 (iv), Telephones &c.], comprising a body portion 1 and a main diaphragm 2 to which is attached a cup part 3 carrying the carbon electrode 4. The diaphragm is clamped to the body portion by a ring 5 to which the auxiliary silk diaphragm 6 is clamped by a pressed-on perforated cap 7. The fixed carbon electrode 8 is seated in a holder 9, the shank of which is secured in a bi-metallic disc 10 by a nut 13, the disc 10 being insulated from the body 1 by a washer 11 and clamped thereon by an insulating cap 12. The disc 10, consisting of two superposed plates of metals having different coefficients of expansion, such as copper and nickel or brass and steel, is inserted so that on being heated it tends to assume an inverted bowl shape, moving the holder 9 inwards. Layers of silk rings 15 form a closure for the front of the granule chamber. According to a modification Fig. 2, a metal diaphragm 16 similar in shape to the diaphragm 2 but inserted in a reverse direction is provided and is clamped in a recess in the body portion by a screwed ring 17, the cup 3 being conically shaped. With this construction on heat being developed in the granules, expansions occur first in the central parts of the diaphgrams and these being coned in opposite directions, the effects of their respective expansions on the moving electrode practically neutralize each other. The body 1 heats up more slowly and its expansion would give rise to a decrease in pressure between the granules. The expansion of that part of the body 1 to which the edge of diaphragm 16 is clamped, however, causes a change of shape of this diaphragm by which its apex is lowered and the pressure tends to be restored. The electrode 8 is also moved inwardly due to the distortion of disc 10. Fig. 3 shows a transmitter of the type described in Specification 308,638, [Class 40 (iv), Telephones &c.], in which the diaphragm consists of two disc parts 18 positioned in a central boss 19 and clamped at their outer edges to a flexible ring 20 which is itself clamped to the holder 21 by a ring 22. The holder 21 is screwed into an apertured fitting 24 and has secured therein a casing 26, the lower end of which is closed by a, mica disc 28 clamped thereto by ring 29. The disc 28 carries the moving electrode 30 which rests on a felt washer 31 the whole being secured by nuts 32, 45 to one end of a rod 46, the other end of which is secured to boss 19. The fixed annular electrode 44, which is secured in the casing 26 by nut 47, is of bimetallic material and is so assembled that, on heat being applied to it, it tends to assume a bowl shape so that the inner circle of the annulus moves towards the moving electrode 30. A mica washer 33, felt washer 48, and nut 39 are arranged above the nut 47. If desired, the disc 28 mav also be made of bimetallic material in which case the ring 29 is made of insulating material and a mica washer is inserted between disc 28 and the casing 26. Fig. 4 shows a transmitter similar to that shown in Fig. 1, but in which the casing 1 contains a bimetallic slit cup 34 which is coated or lined internally with insulating material and rests at the bottom on an insulating washer 37, the tongues formed in the cup tending to turn inwardly under the action of heat whereby the granules are compressed. The movable and fixed electrodes 35, 36 are also bimetallic discs. In all the constructions, the bimetallic discs may be replaced by ordinary metal or mica discs which are initially bowed outwardly so that as the granule chamber expands they tend to flatten.