_Variation of Electrical Pressure._ Variation of the pressure of the
source is a conceivable way of transmitting speech. To utilize it,
would require that the vibrations of the diaphragm cause the
electromotive force of a battery or machine to vary in harmony with
the sound waves. So far as we are informed this method never has come
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