Variable Resistance. As already pointed out in Chapter II, the
variable-resistance method of producing current waves, corresponding
to sound waves for telephonic transmission, is the one that lends
itself most readily to practical purposes. Practically all telephone
transmitters of today employ this variable-resistance principle. The
reason for the adoption of this method instead of the other possible
ones is that the devices acting on this principle are capable, with
great simplicity of construction, of producing much more powerful
results than the others. Their simplicity is such as to make them
capable of being manufactured at low cost and of being used
successfully by unskilled persons.

Materials. Of all the materials available for the
variable-resistance element in telephone transmitters, carbon is by
far the most suitable, and its use is well nigh universal. Sometimes
one of the rarer metals, such as platinum 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.