on 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
called the _local circuit_.

Let us observe what is the advantage of this arrangement over the
case of Fig. 10. Using the same values of resistance in the
transmitter and line, assume the local circuit apart from the
transmitter to have a fixed resistance of 5 ohms. The limits of
variations in the local circuit, therefore, are 10 and 55 ohms, thus
making the maximum 5.5 times the minimum, or an increase of 450 per
cent as against 4.5 per cent in the case of Fig. 10. The changes,
therefore, are 100 times as great.

[Illustration: Fig. 12. Conventional Diagram of Talking Circuit]

The relation between the windings of the induction coil in this
practice are such that the secondary winding contains many more turns
than the primary winding. Changes in the circuit of the primary
winding produce potentials in the secondary winding correspondingly
higher than the potentials producing them. These secondary potentials
depend upon the _ratio_ of turns in the two windings and therefore,
within close limits, may be chosen as wished. High potentials in the
secondary winding are admirably adapted to transmit currents in a
high-resistance line, for exactly the same reason that long-distance
power transmission meets with but one-quarter of one kind of loss when
the sending potential is doubled, one-hundredth of that loss when it
is raised tenfold, and similarly. The induction coil, therefore,
serves the double purpose of a step-up transformer to limit line
losses and a device for vastly increasing the range of change in the
transmitter circuit.

Fig. 13 is offered to remind the student of the action of an induction
coil or transformer in whose primary circuit a direct current is
increased and decreased. An increase of current in the local winding
produces an impulse of _opposite_ direction in the turns of the
secondary winding; a decrease of current in the local winding produces
an impulse of _the same_ direction