avy brass cup _4_. The front
electrode is mounted on the rear face of a stud. Clamped against the
head of this stud, by a screw-threaded clamping ring _7_, is a mica
washer, or disk _6_. The center portion of this mica washer is
therefore rigid with respect to the front electrode and partakes of
its movements. The outer edge of this mica washer is similarly clamped
against the front edge of the cup _4_, a screw-threaded ring _9_
serving to hold the edge of the mica rigidly against the front of the
cup. The outer edge of this washer is, therefore, rigid with respect
to the rear electrode, which is fixed. Whatever relative movement
there is between the two electrodes must, therefore, be permitted by
the flexing of the mica washer. This mica washer not only serves to
maintain the electrodes in their normal relative positions, but also
serves to close the chamber which contains the electrodes, and,
therefore, to prevent the granular carbon, with which the space
between the electrodes is filled, from falling out.

[Illustration: Fig. 40. White Solid-Back Transmitter]

The cup _4_, containing the electrode chamber, is rigidly fastened
with respect to the body of the transmitter by a rearwardly projecting
shank held in a bridge piece _8_ which is secured at its ends to the
front block. The needed rigidity of the rear electrode is thus
obtained and this is probably the reason for calling the instrument
the _solid-back_. The front electrode, on the other hand, is fastened
to the center of the diaphragm by means of a shank on the stud, which
passes through a hole in the diaphragm and is clamped thereto by two
small nuts. Against the rear face of the diaphragm of this transmitter
there rest two damping springs. These are not shown in Fig. 40 but are
in Fig. 41. They are secured at one end to the rear flange of the
front casting _1_, and bear with their other or free ends against the
rear face of the diaphragm. The damping springs are prevented from
coming into actual contact with the diaphragm by small insulating
pads. The purpose of the damping springs is to reduce the
sensitiveness of the diaphragm to extraneous sounds. As a result, the
White transmitter does not pick up all of the sounds in its vicinity
as readily as do the more sensitive transmitters, and thus the
transmission is not interfered with by extraneous noises. On the other
hand, the provision of these heavy damping springs makes it necessary
that this transmitter