which is not affected by heat or moisture. As in the transmitters
previously described, the electrodes are firmly soldered to brass
disks which have solid studs extending from their centers. In the case
of both the front and the rear electrodes, a mica disk is placed over
the supporting stud and held in place by a brass hub which has a
base of the same size as the electrode. The carbon-chamber wall
consists of a brass ring to which are fastened the mica disks of the
front and the back electrodes by means of brass collars clamped over
the edge of the mica and around the rim of the brass ring forming the
chamber.

[Illustration: MAIN OFFICE BUILDING, BERKELEY, CALIFORNIA
Containing Automatic Equipment, Forming Part of Larger System
Operating in San Francisco and Vicinity.
Bay Cities Home Telephone Company.]

Electrodes. The electrode plates of nearly all modern transmitters
are of specially treated carbon. These are first copper-plated and
soldered to their brass supporting disks. After this they are turned
and ground so as to be truly circular in form and to present
absolutely flat faces toward each other. These faces are then highly
polished and the utmost effort is made to keep them absolutely clean.
Great pains are taken to remove from the pores of the carbon, as well
as from the surface, all of the acids or other chemicals that may have
entered them during the process of electroplating them or of soldering
them to the brass supporting disk. That the two electrodes, when
mounted in a transmitter, should be parallel with each other, is an
item of great importance as will be pointed out later.

In a few cases, as previously stated, gold or platinum has been