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
must always be able to speak into the transmitter while seated before
the switchboard, and yet allow both of her hands to be free for use.
This was formerly accomplished by suspending an ordinary
granular-carbon transmitter in front of the operator, but a later
development has resulted in the adoption of the so-called breast
transmitter, shown in Fig. 47. This is merely an ordinary
granular-carbon transmitter mounted on a plate which is strapped on
the breast of the operator, the transmitter being provided with a long
curved mouthpiece which projects in such a manner as to lie just in
front of the operator's lips. This device has the advantage of
automatically following the operator in her movements. The breast
transmitter shown in Fig. 47, is that of the Dean Electric Company.

[Illustration: Fig. 48. Transmitter Symbols]

Conventional Diagram. There are several common ways of illustrating
transmitters in diagrams of circuits in which they are employed. The
three most common ways are shown in Fig. 48. The one at the left is
supp