Still another type of repeating coil is illustrated in diagram in Fig. 112, and in view in Fig. 113.  This coil, like the impedance coil shown in Fig. 104, comprises a core made up of a bundle of soft iron wires wound into the form of a ring.  It is usually provided with two primary windings placed opposite each other upon the core, and with two secondary windings, 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 conductor employed.  The resistance of conductors is subject to change by changes in temperature.  While nearly all metals show an increase, carbon shows a decrease in its resistance when heated.