s, one branch passing through
an impedance coil _7_ and the other through the receiver and the
condenser _8_, in series. The steady current from the central-office
battery finds ready path through the transmitter and the impedance
coil, but is prevented from passing through the receiver by the
barrier set up by the condenser _8_. Voice currents, however, coming
over the line to the station, find ready path through the receiver and
the condenser but are barred from passing through the impedance coil
by virtue of its high impedance.

In considering the action of the station as a transmitting station,
the variations set up by the transmitter pass through the condenser
and the receiver at the same station, while the steady current which
supplies the transmitter passes through the impedance coil. Impedance
coils used for this purpose are made of low ohmic resistance but of a
comparatively great number of turns, and, therefore, present a good
path for steady currents and a difficult path for voice currents. This
divided circuit arrangement employed by the Kellogg Company is one of
the very simple ways of eliminating direct currents from the receiver
path, at the same time allowing the free passage of voice currents.

[Illustration: Fig. 133. Dean System]

_Dean Substation Arrangement._ In marked contrast to the scheme for
keeping steady current out of the receiver circuit employed by the
Kellogg Company, is that shown in Fig. 133, which has been largely
used by the Dean Electric Company, of Elyria, Ohio. The central-office
arrangement in this case is that using the split repeating coil, which
needs no further description. The substation arrangement, however, is
unique and is a beautiful example of what can be done in the way of
preventing a flow of current through a path without in any way
insulating that path or placing any barrier in the way of the current.
It is an example of the prevention of the direct flow of current
through the receiver by so arranging the circuits that there will
always be an equal potential on each side of it, and, therefore, no
tendency for current to flow through it.

In this substation arrangement four coils of wire--_1_, _2_, _3_, and
_4_--are so arranged as to be connected in the circuit of the line,
two in series and two in multiple. The current flowing from the
battery at the central office, after passing through the transmitter,
divides between the two paths containing, respectively, the