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]

Switchboard Transmitter. Another special adaptation of the telephone
transmitter is that for use of telephone operators at central-office
switchboards. The requirements in this case are such that the operator