Acousticon Transmitter.  In Fig. 46 is shown a transmitter adapted for such use.  This has been termed by its makers the acousticon transmitter.  Like all the transmitters previously discussed, this is of the variable-resistance type, but it differs from them all in that it has no damping springs; in that carbon balls are substituted for carbon granules; and in that the diaphragm itself serves as the front electrode.

This transmitter consists of a cup 1, into which is set a cylindrical block 2, in one face of which are a number of hemispherical recesses.  The diaphragm 3 is made of thin carbon and is so placed in the transmitter as to cover the openings of the recesses in the carbon block, and lie close enough to the carbon block, without engaging it, to prevent the carbon particles from falling out.  The diaphragm thus serves as the front electrode and the carbon block as the rear electrode.  The recesses in the carbon block are about two-thirds filled with small carbon balls, which are about the size of fine sand.  The front piece 4 of the transmitter is of sheet metal and serves to hold the diaphragm in place.  To admit the sound waves it is provided with a circular opening opposite to and about the size of the rear electrode block.  On this front piece are mounted the two terminals of the transmitter, connected respectively to the two electrodes, terminal 5 being insulated from the front piece and connected by a thin metal strip with the diaphragm, while terminal 6 is mounted directly on the front piece and connected through the cup 1 with the carbon block 2, or back electrode of the transmitter.

[Illustration:  Fig 46.  Acousticon Transmitter]

When this transmitter is used in connection with outfits for the deaf, it is placed in a hard rubber containing case, consisting of a hollow cylindrical piece 7, which has fastened to it a cover 8.  This cover has a circular row of openings or holes near its outer edge, as shown at 9, through which the sound waves may pass to the chamber within, and thence find their way through the round hole in the center of the front plate 4 to the diaphragm 3.  It is probable also that the front face of the cover 8 of the outer case vibrates, and in this way also causes sound waves to impinge against the diaphragm.  This arrangement provides a large receiving surface for the sound waves, but, owing to the fact that the openings in the containing case are not opposite the opening in the transmitter proper, the sound waves do not impinge directly against the diaphragm.  This peculiar arrangement is probably the result of an endeavor to prevent the transmitter from being too strongly actuated by violent sounds close to it.  Instruments of this kind are very sensitive and under proper conditions are readily responsive to words spoken in an ordinary tone ten feet away.

[Illustration:  Fig. 47.  Switchboard Transmitter]