By bringing out each terminal of each winding
By bringing out each terminal of each winding, eight in all, as shown in this figure, great latitude of connection is provided for, since
the windings may be connected in circuit in any desirable way, either
by connecting them together in pairs to form virtually a primary and a
secondary, or, as is frequently the case, to split the primary and the
secondary, connecting a battery between each pair of windings.
[Illustration: Fig. 109. Repeating Coil]
[Illustration: Fig. 110. Repeating Coil]
Fig. 111 illustrates in section a commercial type of coil designed
for talking through only. This coil is provided with four windings of
1,357 turns each, and when used for local battery work the coils are
connected in pairs in series, thus giving a resistance of about 190
ohms in each half of the repeating coil. The core of this coil
consists of a bundle of soft iron wires, and the shell which forms the
return path for the magnetic lines is of very soft sheet iron. This
shell is drawn into cup shape and its open end is closed, after the
coil is inserted, by the insertion of a soft iron head, as indicated.
As in the case of the coil shown in Figs. 109 and 110, eight terminals
are brought out on this coil, thus providing the necessary flexibility
of connection.
[Illustration: Fig. 111. Repeating Coil]
[Illustration: Fig. 112. Diagram of Toroidal Repeating Coil]
[Illustration: Fig. 113. Toroidal Repeating Coil]
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.
The temperature coefficient of a conductor is a factor by which the
resistance of the conductor at a given temperature must be multiplied
in order to determine the change in resistance of that conductor
brought about by a rise in temperature of one degree.