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 induction coil related to a carbon transmitter, a
battery, and a receiver. Fig. 12 shows exactly the same arrangement,
using conventional signs. The winding of the induction coil which is
in series with the transmitter and the battery is called the primary
winding; the other is called the secondary winding. In the arrangement
of Figs. 11 and 12 the battery has no metallic connection with the
line, so that it is called a _local battery_. The circuit containing
the battery, transmitter, and primary winding of the induction coil is