rding to the Kellogg two-battery system is
indicated in Fig. 140, which needs no further explanation in view of
the description of the preceding figures. It is interesting to note in
this case that the left-hand battery serves only the left-hand lines
and the right-hand battery only the right-hand lines. As this is
worked out in practice, the left-hand battery is always connected to
those lines which originate a call and the right-hand battery always
to those lines that are called for. The energy supplied to a calling
line is always, therefore, from a different source than that which
supplies a called line.

[Illustration: Fig. 140. Two Sources for Many Lines]

[Illustration: Fig. 141. Current Supply from Distant Point]

_Current Supply from Distant Point._ Sometimes it is convenient to
supply current to a group of lines centering at a certain point from a
source of current located at a distant point. This is often the case
in the so-called private branch exchange, where a given business
house or other institution is provided with its own switchboard for
interconnecting the lines leading to the various telephones of that
concern or institution among themselves, and also for connecting them
with lines leading to the city exchange. It is not always easy or
convenient to maintain at such private switchboards a separate battery
for supplying the current needed by the local exchange.

In such cases the arrangement shown in Fig. 141 is sometimes employed.
This shows two pairs of lines connected by the impedance-coil system
with common terminals _1_ and _2_, between which ordinarily the common
battery would be connected. Instead of putting a battery between these
terminals, however, at the local exchange, a condenser of large
capacity is connected between them and from these terminals circuit
wires _3_ and _4_ are led to a battery of suitable voltage at a
distant central office. The condenser in this case is used to afford a
short-circuit path for the voice currents that leak from one side of
one pair of lines to the other, through the impedance coils bridged
across the line. In this way the effect of the necessarily high
resistance in the common leads _3_ and _4_, leading to the storage
battery, is overcome and the tendency to cross-talk between the
various pairs of connected lines is eliminated. Frequently, instead of
employing this arrangement, a storage battery of small capacity will
be connected between the terminals