ndings, one over each primary. In practice these two
primary windings are connected in one circuit and the two secondaries
in another. This is the standard repeating coil now used by the Bell
companies in their common-battery cord circuits.

[Illustration: THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY,
MISSOURI]

[Illustration: Fig. 114. Symbol of Induction Coil]

Conventional Symbols. The ordinary symbol for the induction coil
used in local battery work is shown in Fig. 114. This consists merely
of a pair of parallel zig-zag lines. The primary winding is usually
indicated by a heavy line having a fewer number of zig-zags, and the
secondary by a finer line having a greater number of zig-zags. In this
way the fact that the primary is of large wire and of comparatively
few turns is indicated. This diagrammatic symbol may be modified to
suit almost any conditions, and where a tertiary as well as a
secondary winding is provided it may be shown by merely adding another
zig-zag line.

[Illustration: Fig. 115. Repeating-Coil Symbols]

The repeating coil is indicated symbolically in the two diagrams of
Fig. 115. Where there is no necessity for indicating the internal
connections of the coil, the symbol shown in the left of this figure
is usually employed. Where, however, the coil consists of four
windings rather than two and the method of connecting them is to be
indicated, the symbol at the right hand is employed. In Fig. 116
another way of indicating a four-winding repeating coil or induction
coil is shown. Sometimes such windings may be combined by connection
to form merely a primary and a secondary winding, and in other cases
the four windings all act separately, in which case one may be
considered the primary and the others, respectively, the secondary,
tertiary, and quaternary.

[Illustration: Fig. 116. Symbol of Four-Winding Repeating Coil]

Where the toroidal type of repeating coil is employed, the diagram of
Fig. 112, already referred to, is a good symbolic representation.

CHAPTER XI

NON-INDUCTIVE RESISTANCE DEVICES

It is often desired to introduce simple ohmic resistance into
telephone circuits, in order to limit the current flow, or to create
specific differences of potential at given points in the circuit.

Temperature Coefficient. The design or selection of resistance
devices for various purposes frequently involves the consideration of
the effect of temperature on the resistance of the c