or gold, is to be found in
commercial transmitters as part of the resistance-varying device, but,
even when this is so, it is always used in combination with carbon in
some form or other. Most of the transmitters in use, however, depend
solely upon carbon as the conductive material of the
variable-resistance element.

Arrangement of Electrodes. Following the principles pointed out by
Hughes, the transmitters of today always employ as their
variable-resistance elements one or more loose contacts between one or
more pairs of electrodes, which electrodes, as just stated, are
usually of carbon. Always the arrangement is such that the sound waves
will vary the intimacy of contact between the electrodes and,
therefore, the resistance of the path through the electrodes.

A multitude of arrangements have been proposed and tried. Sometimes a
single pair of electrodes has been employed having a single point of
loose contact between them. These may be termed single-contact
transmitters. Sometimes the variable-resistance element has included a
greater number of electrodes arranged in multiple, or in series, or in
series-multiple, and these have been termed multiple-electrode
transmitters, signifying a plurality of electrodes. A later
development, an outgrowth of the multiple-electrode transmitter, makes
use of a pair of principal electrodes, between which is included a
mass of finely divided carbon in the form of granules or small spheres
or pellets. These, regardless of the exact form of the carbon
particles, are called granular-carbon transmitters.

[Illustration: Fig. 38. Blake Transmitter]

Single Electrode. _Blake_. The most notable example of the
single-contact transmitter is the once familiar Blake instrument. At
one time this formed a part of the standard equipment of almost every
telephone in the United States, and it was also largely used abroad.
Probably no transmitter has ever exceeded it in clearness of
articulation, but it was decidedly deficient in power in comparison
with the modern transmitters. In this instrument, which is shown in
Fig. 38, the variable-resistance contact was that between a carbon and
a platinum electrode. The diaphragm _1_ was of sheet iron mounted, as
usual in later transmitters, in a soft rubber gasket _2_. The whole
diaphragm was mounted in a cast-iron ring _3_, supported on the inside
of the box containing the entire instrument. The front electrode _4_
was mounted on a light spring _5_,