e
into practical use.

_Variation of Resistance._ Variation of resistance proportional to the
vibrations of the diaphragm is the method which has produced the
present prevailing form of transmission. Professor Bell's Centennial
exhibit contained a water-resistance transmitter. Dr. Elisha Gray
also devised one. In both, the diaphragm acted to increase and
diminish the distance between two conductors immersed in water,
lowering and raising the resistance of the line. It later was
discovered by Edison that carbon possesses a peculiarly great property
of varying its resistance under pressure. Professor David E. Hughes
discovered that two conducting bodies, preferably of rather poor
conductivity, when laid together so as to form a _loose contact_
between them, possessed, in remarkable degree, the ability to vary the
resistance of the path through them when subject to such vibrations as
would alter the _intimacy of contact_. He thus discovered and
formulated the principles of _loose contact_ upon which the operation
of all modern transmitters rests. Hughes' device was named by him a
"microphone," indicating a magnification of sound or an ability to
respond to and make audible minute sounds. It is shown in Fig. 8.
Firmly attached to a board are two carbon blocks, shown in section in
the figure. A rod of carbon with cone-shaped ends is supported loosely
between the two blocks, conical depressions in the blocks receiving
the ends of the rod. A battery and magneto receiver are connected in
series with the device. Under certain conditions of contact, the
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-