carbon type universal now. Other
types popular at other times and in particular places use solid rods
or blocks of carbon having many points of contact, though not in a
powdered or granular form. Fig. 9 shows an example of each of the
general forms of transmitters.

The use of granular carbon as a transmitter material has extended
greatly the radius of speech, and has been a principal contributing
cause for the great spread of the telephone industry.

[Illustration: Fig. 9. General Types of Transmitters]

_Superiority._ The superiority of carbon over other resistance-varying
materials for transmitters is well recognized, but the reason for it
is not well known. Various theories have been proposed to explain why,
for example, the resistance of a mass of carbon granules varies with
the vibrations or compressions to which they are subjected.

Four principal theories respectively allege:

First, that change in pressure actually changes the specific
resistance of carbon.

Second, that upon the surface of carbon bodies exists some gas in
some form of attachment or combination, variations of pressure
causing variations of resistance merely by reducing the thickness
of this intervening gas.

Third, that the change of resistance is caused by variations in
the length of electrical arcs between the particles.

Fourth, that change of pressure changes the area of contact, as
is true of solids generally.

One may take his choice. A solid carbon block or rod is not found to
decrease its resistance by being subjected to pressure. The gas theory
lacks experimental proof also. The existence of arcs between the
granules never has been seen or otherwise observed under normal
working conditions of a transmitter; when arcs surely are
experimentally established between the granules the usefulness of the
transmitter ceases. The final theory, that change of pressure changes
area of surface contact, does not explain why other conductors than
carbon are not good materials for transmitters. This, it may be
noticed, is just what the theories set out to make clear.

There are many who feel that more experimental data is required before
a conclusive and satisfactory theory can be set up. There is need of
one, for a proper theory often points the way for effective advance in
practice.

Carbon and magneto transmitters differ wholly in their methods of
action. The magneto transmitter _produ