In this transmitter, as in the transmitter shown in Fig. 42, all of the working parts are insulated from the exposed metal casing.  The diaphragm is insulated from the front of the instrument by means of a washer 4 of impregnated cloth, as indicated.  The rear electrode is insulated from the other portions of the instrument by means of the mica washer and by means of the insulation between the block 9 and the bridge 5.  The terminal for the rear electrode is mounted on the block 9, while the terminal for the front electrode, shown at 10, is mounted on, but insulated from, the bridge.  This terminal 10 is connected with the diaphragm and therefore with the front electrode by means of a thin, flexible metallic connection.  This transmitter is provided with damping springs similar to those of the White instrument.

It is claimed by advocates of this type of instrument that, in addition to the ordinary action due to the compression and decompression of the granular carbon between the electrodes, there exists another action due to the agitation of the granules as the chamber is caused to vibrate by the sound waves.  In other words, in addition to the ordinary action, which may be termed the piston action between the electrodes, it is claimed that the general shaking-up effect of the granules when the chamber vibrates produces an added effect.  Certain it is, however, that transmitters of this general type are very efficient and have proven their capability of giving satisfactory service through long periods of time.

Another interesting feature of this instrument as it is now manufactured is the use of a transmitter front that is struck up from sheet metal rather than the employment of a casting as has ordinarily been the practice.  The formation of the supporting lug for the transmitter from the sheet metal which forms the rear casing or shell of the instrument is also an interesting feature.

Automatic Electric Company Transmitter.  The transmitter of the Automatic Electric Company, of Chicago, shown in Fig. 44, is of the same general type as the one just discussed, in that the electrode chamber is mounted on and vibrates with the diaphragm instead of being rigidly supported on the bridge as in the case of the White or solid-back type of instrument.  In this instrument the transmitter front 1 is struck up from sheet metal and contains a rearwardly projecting flange, carrying an internal screw thread.  A heavy inner cup 2, together with the diaphragm 3, form an enclosure containing the electrode chamber.  The diaphragm is, in this case, permanently secured at its edge to the periphery of the inner cup 2 by a band of metal 4 so formed as to embrace the edges of both the cup and the diaphragm and permanently lock them together.  This inner chamber is held in place in the transmitter front 1 by means of a lock ring 5 externally screw-threaded to engage the internal screw-thread on the flange on the front.  The electrode chamber proper is made in the form of a cup, rigidly secured to the diaphragm so as to move therewith, as clearly indicated.  The rear electrode is mounted on a screw-threaded stud carried in a block which is fitted to a close central opening in the cup 2.