shall be spoken into directly by the user.

[Illustration: Fig. 41. White Solid-Back Transmitter]

The action of this transmitter is as follows: Sound waves are
concentrated against the center of the diaphragm by the mouth-piece,
which is of the familiar form. These waves impinge against the
diaphragm, causing it to vibrate, and this, in turn, produces similar
vibrations in the front electrode. The vibrations of the front
electrode are permitted by the elasticity of the mica washer _6_. The
rear electrode is, however, held stationary within the heavy chambered
block _4_ and which in turn is held immovable by its rigid mounting.
As a result, the front electrode approaches and recedes from the rear
electrode, thus compressing and decompressing the mass of granular
carbon between them. As a result, the intimacy of contact between the
electrode plates and the granules and also between the granules
themselves is altered, and the resistance of the path from one
electrode to the other through the mass of granules is varied.

New Western Electric Transmitter. The White transmitter was the
prototype of a large number of others embodying the same features of
having the rear electrode mounted in a stationary cup or chamber and
the front electrode movable with the diaphragm, a washer of mica or
other flexible insulating material serving to close the front of the
electrode chamber and at the same time to permit the necessary
vibration of the front electrode with the diaphragm.

[Illustration: Fig. 42. New Western Electric Transmitter]

One of these transmitters, embodying these same features but with
modified details, is shown in Fig. 42, this being the new transmitter
manufactured by the Western Electric Company. In this the bridge of
the original White transmitter is dispensed with, the electrode
chamber being supported by a pressed metal cup _1_, which supports the
chamber as a whole. The electrode cup, instead of being made of a
solid block as in the White instrument, is composed of two portions, a
cylindrical or tubular portion _2_ and a back _3_. The cylindrical
portion is externally screw-threaded so as to engage an internal screw
thread in a flanged opening in the center of the cup _1_. By this
means the electrode chamber is held in place in the cup _1_, and by
the same means the mica washer _4_ is clamped between the flange in
this opening and the tubular portion _2_ of the electrode chamber. The
front electrode is car