arrangement is extraordinarily sensitive to small sounds and
approaches an ability indicated by its name. Its practical usefulness
has been not as a serviceable speech transmitter, but as a stimulus to
the devising of transmitters using carbon in other ways. Variation of
the resistance of metal conductors and of contact between metals has
served to transmit voice currents, but no material approaches carbon
in this property.

[Illustration: Fig. 8. Hughes' Microphone]

Carbon. _Adaptability._ The application of carbon to use in
transmitters has taken many forms. They may be classified as those
having a single contact and those having a plurality of contacts; in
all cases, the _intimacy of contact_ is varied by the diaphragm
excursions. An example of the single-contact type is the Blake
transmitter, long familiar in America. An example of the
multiple-contact type is the loose-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