slipped on the surface, and, being connected to a diaphragm, made
it vibrate and repeat the original sounds. This "electro-
motograph" receiver was, however, given up, and a combination of
the Edison transmitter and the Bell receiver came into use.

At the end of 1877 Professor D. E. Hughes, a distinguished
Welshman, inventor of the printing telegraph, discovered that any
loose contact between two conductors had the property of
transmitting sounds by varying the strength of an electric current
passing through it. Two pieces of metal--for instance, two nails
or ends of wire--when brought into a loose or crazy contact under
a slight pressure, and traversed by a current, will transmit
speech. Two pieces of hard carbon are still better than metals,
and if properly adjusted will make the tread of a fly quite
audible in a telephone connected with them. Such is the famous
"microphone," by which a faint sound can be magnified to the ear.

Figure 57 represents what is known as the "pencil" microphone, in
which M is a pointed rod of hard carbon, delicately poised between
two brackets of carbon, which are connected in circuit with a
battery B and a Bell telephone T. The joints of rod and bracket
are so sensitive that the current flowing across them is affected
in strength by the slightest vibration, even the walking of an
insect. If, therefore, we speak near this microphone, the sonorous
waves, causing the pencil to vibrate, will so vary the current in
accordance with them as to reproduce the sounds of the voice in
the telephone.

The true nature of the microphone is not yet known, but it is
evident that the air or ether between the surfaces in contact