[Illustration:  Fig. 107.  Induction Coil]

[Illustration:  Fig. 108.  Section of Induction Coil]

Use and Advantage. The use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters.  Such induction coils are nearly always of the open magnetic circuit type, consisting of a long, straight core comprised of a bundle of small annealed iron wires, on which is wound a primary of comparatively coarse wire and having a small number of turns, and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns.  A view of such a coil mounted on a base is shown in Fig. 107, and a sectional view of a similar coil is shown in Fig. 108.  The method of bringing out the winding terminals is clearly indicated in this figure, the terminal wires 2 and 4 being those of the primary winding and 1 and 3 those of the secondary winding.  It is customary to bring out these wires and attach them by solder to suitable terminal clips.  In the case of the coil shown in Fig. 108 these clips are mounted on the wooden heads of the coil, while in the design shown in Fig. 107 they are mounted on the base, as is clearly indicated.

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 association 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 necessary to ring through them as well as talk through them.  By this is meant that it is necessary 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 thousand 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 versa.  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.