coils _1_
and _3_ and the coils _2_ and _4_. The receiver is connected between
the junction of the coils _2_ and _4_ and that of _1_ and _3_. The
resistances of the coils are so chosen that the drop of potential
through the coil _2_ will be equal to that through the coil _1_, and
likewise that through the coil _4_ will be equal to that through the
coil _3_. As a result, the receiver will be connected between two
points of equal potential, and no direct current will flow through it.
How, then, do voice currents find their way through the receiver, as
they evidently must, if the circuit is to fulfill any useful function?
The coils _2_ and _3_ are made to have high impedance, while _1_ and
_4_ are so wound as to be non-inductive and, therefore, offer no
impedance save that of their ohmic resistance. What is true,
therefore, of direct currents does not hold for voice currents, and as
a result, the voice currents, instead of taking the divided path which
the direct currents pursued, are debarred from the coils _2_ and _3_
by their high impedance and thus pass through the non-inductive coil
_1_, the receiver, and the non-inductive coil _4_.

This circuit employs a Wheatstone-bridge arrangement, adjusted to a
state of balance with respect to direct currents, such currents being
excluded from the receiver, not because the receiver circuit is in any
sense opaque to such direct currents, but because there is no
difference of potential between the terminals of the receiver circuit,
and, therefore, no tendency for current to flow through the receiver.
In order that fluctuating currents may not, for the same reason, be
caused to pass by, rather than through, the receiver circuit, the
diametrically-opposed arms of the Wheatstone bridge are made to
possess, in large degree, self-induction, thereby giving these two
arms a high impedance to fluctuating currents. The conditions which
exist for direct currents do not, therefore, exist for fluctuating
currents, and it is this distinction which allows alternating currents
to pass through the receiver and at the same time excludes direct
currents therefrom.

In practice, the coils _1_, _2_, _3_, and _4_ of the Dean substation
circuit are wound on the same core, but coils _1_ and _4_--the
non-inductive ones--are wound by doubling the wire back on itself so
as to neutralize their self-induction.

_Stromberg-Carlson._ Another modification of the central-office
arrangement and also of the su