ths by virtue of the impedance.

[Illustration: Fig. 137. Current Supply over Parallel Limbs of Line]

An objection to this form of current supply and to other similar
forms, wherein the transmitter current is fed over the two sides of
the line in multiple with a ground return, is that the ground-return
circuit formed by the two sides of the line in multiple is subject to
inductive disturbances from other lines in the same way as an ordinary
grounded line is subject to inductive disturbance. The current-supply
circuit is thus subject to external disturbances and such disturbances
find their way into the metallic circuit and, therefore, through the
instruments by means of the electromagnetic induction between the
primary and the secondary coils at the substations.

Another interesting method of current supply from a central-office
battery is shown in Fig. 137. This, like the circuit just considered,
feeds the energy to the subscriber's station over the two sides of the
line in multiple with a ground return. In this case, however, a local
circuit is provided at the substation, in which is placed a storage
battery _1_ and the primary _2_ of an induction coil, together with
the transmitter. The idea in this is that the current supply from the
central office will pass through the storage battery and charge it.
Upon the use of the transmitter, this storage battery acts to supply
current to the local circuit containing the transmitter and the
primary coil _2_ in exactly the same manner as in a local battery
system. The fluctuating current so produced by the action of the
transmitter in this local circuit acts on the secondary winding _3_ of
the induction coil, and produces therein alternating currents which
pass to the central office and are in turn repeated to the distant
station.

_Supply Many Lines from Common Source._ We come now to the
consideration of the arrangement by which a single battery may be made
to supply current at the central office to a large number of pairs of
connected lines simultaneously. Up to this point in this discussion it
has been shown only how each battery served a single pair of connected
lines and no others.

Repeating Coil:--In Fig. 138 is shown how a single battery supplies
current simultaneously to four different pairs of lines, the lines of
each pair being connected for conversation. It is seen that the pairs
of lines shown in this figure are arranged in each case in accordance
with the