is own receiver, unconsciously
assumes that he is talking too loud and, therefore, lowers his voice,
sometimes to such an extent that it will not properly reach the
distant station.

[Illustration: Fig. 131. Bridging Battery with Impedance Coils]

_Bridging Battery with Impedance Coils._ The method of feeding current
to the line from the common battery, shown in Fig. 130, is called the
"split repeating-coil" method. As distinguished from this is the
impedance-coil method which is shown in Fig. 131. In this the battery
is bridged across the circuit of the combined lines in series with two
impedance coils, _1_ and _2_, one on each side of the battery. The
steady currents from the battery find ready path through these
impedance coils which are of comparatively low ohmic resistance, and
the current divides and passes in multiple over the circuits of the
two lines. Voice currents, however, originating at either one of the
stations, will not pass through the shunt across the line at the
central office on account of the high impedance offered by these
coils, and as a result they are compelled to pass on to the distant
station and affect the receiver there, as desired.

This impedance-coil method seems to present the advantage of greater
simplicity over the repeating-coil method shown in Fig. 130, and so
far as talking efficiency is concerned, there is little to choose
between the two. The repeating-coil method, however, has the advantage
over this impedance-coil method, because by it the two lines are
practically divided except by the inductive connection between the two
windings, and as a result an unbalanced condition of one of the
connected lines is not as likely to produce an unbalanced condition in
the other as where the two lines are connected straight through, as
with the impedance-coil method. The substation arrangement of Fig. 131
is the same as that of Fig. 130.

[Illustration: Fig. 132. Double-Battery Kellogg System]

_Double Battery with Impedance Coils._ A modification of the
impedance-coil method is used in all of the central-office work of the
Kellogg Switchboard and Supply Company. This employs a combination of
impedance coils and condensers, and in effect isolates the lines
conductively from each other as completely as the repeating-coil
method. It is characteristic of all the Kellogg common-battery systems
that they employ two batteries instead of one, one of these being
connected in all cases with the