w through
the receiver coils in order to give the proper magnetization to the
receiver cores, and this has brought about a return to the very simple
form of substation circuit, which includes the receiver and the
transmitter directly in the circuit of the line. This illustrates well
an occurrence that is frequently observed by those who have
opportunity to watch closely the development of an art. At one time
the conditions will be such as to call for complicated arrangements,
and for years the aim of inventors will be to perfect these
arrangements; then, after they are perfected, adopted, and
standardized, a new idea, or a slight alteration in the practice in
some other respect, will demand a return to the first principles and
wipe out the necessity for the things that have been so arduously
striven for.

[Illustration: Fig. 130. Bridging Battery with Repeating Coil]

_Bridging Battery with Repeating Coil._ As pointed out, the placing of
the battery in series in the line circuit in the central office is not
desirable, and, so far as we are aware, has never been extensively
used. The universal practice, therefore, is to place it in a bridge
path across the line circuit, and a number of arrangements employing
this basic idea are in wide use. In Fig. 130 is shown the standard
arrangement of the Western Electric Company, employed by practically
all the Bell operating companies. In this the battery at the central
office is connected in the middle of the two sides of a repeating coil
so that the current from the battery is fed out to the two connected
lines in multiple.

Referring to the middle portion of this figure showing the
central-office apparatus, _1_ and _2_ may be considered as the two
halves of one side of a repeating coil divided so that the battery may
be cut into their circuit. Likewise, _3_ and _4_ may be considered as
the two halves of the other side of the repeating coil similarly
divided for the same purpose. The windings of this repeating coil are
ordinarily alike; that is, _1_ and _2_ combined have the same
resistance, number of turns, and impedance as _3_ and _4_ combined.
The two sides of this coil are alternately used as primary and
secondary, _1_ and _2_ forming the primary when Station A is talking,
and _3_ and _4_, the secondary; and _vice versa_ when Station B is
talking.

As will be seen, the current flowing from the positive pole of the
battery will divide and flow through the windings _2_ an