On the other hand, for carrying voice currents alone it is usually made with a small amount of iron and with small windings, in order to prevent waste of energy in the core, and to give a high degree of responsiveness with the least amount of distortion of wave form, so that the voice currents will retain as far as possible their original characteristics.  When, therefore, a coil is required to carry both ringing and talking currents, a compromise must be effected.

Types. The form of repeating coil largely used for both ringing and talking through is shown in Fig. 109.  This coil comprises a soft iron core made up of a bundle of wires about .02 inch in diameter, the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core.  The windings of this particular coil are four in number, and contain about 2,400 turns each, and have a resistance of about 60 ohms.  In this coil, when connected for local battery work, the windings are connected in pairs in series, thus forming effectively two windings having about 120 ohms resistance each.  The whole coil is enclosed in a protecting case of iron.  The terminals are brought out to suitable clips on the wooden base, as shown.  An external perspective view of this coil is shown in Fig. 110.  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]