ooden 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. 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