c circuit is shown
in Fig. 129. In thus placing the battery in series in the circuit
between the two stations, as shown in Figs. 128 and 129, it is
obvious that the transmitter at each station is compelled to vary the
resistance of the entire circuit comprising the two lines in series,
in order to affect the receiver at distant stations. This is in effect
making the transmitter circuit twice as long as is necessary, as will
be shown in the subsequent systems considered. Furthermore, the
placing of the battery in series in the circuit of the two combined
lines does not lend itself readily to the supply of current from a
common source to more than a single pair of lines.

[Illustration: Fig. 129. Battery in Series with Two Lines]

_Series Substation Circuit._ The arrangement at the
substations--consisting in placing the transmitter and the receiver in
series in the line circuit, as shown in Figs. 128 and 129--is the
simplest possible one, and has been used to a considerable extent, but
it has been subject to the serious objection, where receivers having
permanent magnets were used, of making it necessary to so connect the
receiver in the line circuit that the steady current from the battery
would not set up a magnetization in the cores of the receiver in such
a direction as to neutralize or oppose the magnetization of the
permanent magnets. As long as the current flowed through the receiver
coils in such a direction as to supplement the magnetization of the
permanent magnets, no harm was usually done, but when the current
flowed through the receiver coils in such a way as to neutralize or
oppose the magnetizing force of the permanent magnets, the action of
the receiver was greatly interfered with. As a result, it was
necessary to always connect the receivers in the line circuit in a
certain way, and this operation was called _poling_.

In order to obviate the necessity for poling and also to bring about
other desirable features, it has been, until recently, almost
universal practice to so arrange the receiver that it would be in the
circuit of the voice currents passing over the line, but would not be
traversed by direct currents, this condition being brought about by
various arrangements of condensers, impedance coils, or induction
coils, as will be shown later. During the year 1909, however, the
adoption by several concerns of the so-called "direct-current"
receiver has made it necessary for the direct current to flo