the early eighties. While this idea forms the basis of all modern
telephone transmitters, yet it did not prevent the almost universal
adoption of the single-contact form of instrument during the next
decade.

Western Electric Solid-Back Transmitter. In the early nineties,
however, the granular-carbon transmitter came into its own with the
advent and wide adoption of the transmitter designed by Anthony C.
White, known as the _White_, or _solid-back_, transmitter. This has
for many years been the standard instrument of the Bell companies
operating throughout the United States, and has found large use
abroad. A horizontal cross-section of this instrument is shown in Fig.
40, and a rear view of the working parts in Fig. 41. The working parts
are all mounted on the front casting _1_. This is supported in a cup
_2_, in turn supported on the lug _3_, which is pivoted on the
transmitter arm or other support. The front and rear electrodes of
this instrument are formed of thin carbon disks shown in solid black.
The rear electrode, the larger one of these disks, is securely
attached by solder to the face of a brass disk having a rearwardly
projecting screw-threaded shank, which serves to hold it and the rear
electrode in place in the bottom of a heavy 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