of such highly sensitive transmitters is found in the special
intercommunicating telephone systems for use between the various
departments or desks in business offices. In these it is desirable
that the transmitter shall be able to respond adequately to sounds
occurring anywhere in a small-sized room, for instance.

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