ces_ current; the carbon
transmitter _controls_ current. The former is an alternating-current
generator; the latter is a rheostat. The magneto transmitter produces
alternating current without input of any electricity at all; the
carbon transmitter merely controls a direct current, supplied by an
external source, and varies its amount without changing its direction.

The carbon transmitter, however, may be associated with other devices
in a circuit in such a way as to _transform_ direct currents into
alternating ones, or it may be used merely to change constant direct
currents into _undulating_ ones, which _never_ reverse direction, as
alternating currents _always_ do. These distinctions are important.

[Illustration: Fig. 10. Battery in Line Circuit]

_Limitations._ A carbon transmitter being merely a resistance-varying
device, its usefulness depends on how much its resistance can vary in
response to motions of air molecules. A granular-carbon transmitter
may vary between resistances of 5 to 50 ohms while transmitting a
particular tone, having the lower resistance when its diaphragm is
driven inward. Conceive this transmitter to be in a line as shown in
Fig. 10, the line, distant receiver, and battery together having a
resistance of 1,000 ohms. The minimum resistance then is 1,005 ohms
and the maximum 1,050 ohms. The variation is limited to about 4.5 per
cent. The greater the resistance of the line and other elements than
the transmitter, the less relative change the transmitter can produce,
and the less loudly the distant receiver can speak.

[Illustration: Fig 11. Battery in Local Circuit]

Induction Coil. Mr. Edison realized this limitation to the use of
the carbon transmitter direct in the line, and contributed the means
of removing it. His method is to introduce an induction coil between
the line and the transmitter, its function being to translate the
variation of the direct current controlled by the transmitter into
true alternating currents.

An induction coil is merely a transformer, and for the use under
discussion consists of two insulated wires wound around an iron core.
Change in the current carried by one of the windings _produces_ a
current in the other. If direct current be flowing in one of the
windings, and remains constant, no current whatever is produced in the
other. It is important to note that it is change, and change only,
which produces that alternating current.

Fig. 11 shows an inducti