in which Edison has utilized the
peculiar property that carbon possesses of altering its resistance
to the passage of current, according to the pressure to which it is
subjected, whether at the surface, or through closer union of the
mass. A loose road with a few inches of dust or pebbles on it offers
appreciable resistance to the wheels of vehicles travelling over it; but
if the surface is kept hard and smooth the effect is quite different.
In the same way carbon, whether solid or in the shape of finely divided
powder, offers a high resistance to the passage of electricity; but
if the carbon is squeezed together the conditions change, with less
resistance to electricity in the circuit. For his quadruplex system,
Mr. Edison utilized this fact in the construction of a rheostat or
resistance box. It consists of a series of silk disks saturated with a
sizing of plumbago and well dried. The disks are compressed by means of
an adjustable screw; and in this manner the resistance of a circuit can
be varied over a wide range.

In like manner Edison developed a "pressure" or carbon relay, adapted
to the transference of signals of variable strength from one circuit to
another. An ordinary relay consists of an electromagnet inserted in the
main line for telegraphing, which brings a local battery and sounder
circuit into play, reproducing in the local circuit the signals sent
over the main line. The relay is adjusted to the weaker currents likely
to be received, but the signals reproduced on the sounder by the agency
of the relay are, of course, all of equal strength, as they depend upon
the local battery, which has only this steady work to perform. In cases
where it is desirable to reproduce the signals in the local circuit with
the same variations in strength as they are received by the relay,
the Edison carbon pressure relay does the work. The poles of the
electromagnet in the local circuit are hollowed out and filled up with
carbon disks or powdered plumbago. The armature and the carbon-tipped
poles of the electromagnet form part of the local circuit; and if the
relay is actuated by a weak current the armature will be attracted
but feebly. The carbon being only slightly compressed will offer
considerable resistance to the flow of current from the local battery,
and therefore the signal on the local sounder will be weak. If, on the
contrary, the incoming current on the main line be strong, the armature
will be strongly attracted