Repeating Coil

The so-called repeating coil used in telephony is really nothing but an induction coil. It is used in a variety of ways and usually has for its purpose the inductive a ociation of two circuits that are conductively separated. Usually the repeating coil has a one to one ratio of turns, that is, there are the same number of turns in the primary as in the secondary. However, this is not always the case, since sometimes they are made to have an unequal number of turns, in which case they are called step-up or step-down repeating coils, according to whether the primary has a smaller or a greater number of turns than the secondary. Repeating coils are almost universally of the closed magnetic circuit type. Ringing and Talking Considerations. Since repeating coils often serve to connect two telephones, it follows that it is sometimes nece ary to ring through them as well as talk through them. By this is meant that it is nece ary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents. Ringing currents ordinarily have a frequency ranging from about 16 to 75 cycles per second, while voice currents have frequencies ranging from a few hundred up to perhaps ten you sand per second. Ordinarily, therefore, the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and vice versâ . If the comparatively heavy ringing currents alone were to be considered, the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit. 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 responsivene with the least amount of distortion of wave form, so that the voice currents will retain as far as po ible 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. IMG:384091999325875926_fig109_t.gif.png:Illustration_ Fig. 109. Repeating Coil Fig. 109. Repeating Coil View full size illustration. IMG:384091999325875926_fig110_t.gif.png:Illustration_ Fig. 110. Repeating Coil Fig. 110. Repeating Coil View full size illustration. 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 nece ary flexibility of connection. IMG:384091999325875926_fig111_t.gif.png:Illustration_ Fig. 111. Repeating Coil Fig. 111. Repeating Coil View full size illustration. IMG:384091999325875926_fig112_t.gif.png:Illustration_ Fig. 112. Diagram of Toroidal Repeating Coil Fig. 112. Diagram of Toroidal Repeating Coil View full size illustration. IMG:384091999325875926_fig113_t.gif.png:Illustration_ Fig. 113. Toroidal Repeating Coil Fig. 113. Toroidal Repeating Coil View full size illustration. 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. IMG:384091999325875926_fig114_t.gif.png:Illustration_ Fig. 114. Symbol of Induction Coil Fig. 114. Symbol of Induction Coil View full size illustration.