itter which it supplies; and second,
those where the battery or other source of current is located at a
distant point from the transmitter, the battery in such cases serving
as a common source of current for the supply of transmitters at a
number of stations.

The advantages of putting the transmitter and the battery which
supplies it with current in a local circuit with the primary of an
induction coil, and placing the secondary of the induction coil in the
line, have already been pointed out but may be briefly summarized as
follows: When the transmitter is placed directly in the _line circuit_
and the line is of considerable length, the current which passes
through the transmitter is necessarily rather small unless a battery
of high potential is used; and, furthermore, the total change in
resistance which the transmitter is capable of producing is but a
small proportion of the total resistance of the line, and, therefore,
the current changes produced by the transmitter are relatively small.
On the other hand, when the transmitter is placed in a _local circuit_
with the battery, this circuit may be of small resistance and the
current relatively large, even though supplied by a low-voltage
battery; so that the transmitter is capable of producing relatively
large changes in a relatively large current.

To draw a comparison between these two general classes of transmitter
current supply, a number of cases will be considered in connection
with the following figures, in each of which two stations connected by
a telephone line are shown. Brief reference to the local battery
method of supplying current will be made in order to make this chapter
contain, as far as possible, all of the commonly used methods of
current supply to transmitters.

[Illustration: A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT]

Local Battery. In Fig. 125 two stations are shown connected by a
grounded line wire. The transmitter of each station is included in a
low-resistance primary circuit including a battery and the primary
winding of an induction coil, the relation between the primary
circuits and the line circuits being established by the inductive
action between the primary and the secondary windings of induction
coils, the secondary in each case being in the line circuits with the
receivers.

[Illustration: Fig. 125. Local-Battery Stations with Grounded Circuit]

Fig. 126 shows exactly the same arrangement but with a metallic
circ