Swedish or Norway iron wire about
Swedish or Norway iron wire about .02 of an inch in diameter. The
diameter and the length of the coil, and the relation between the
number of turns in the primary and in the secondary, and the
mechanical construction of the coil, are all matters which are subject
to very wide variation in practice. While the proper relationship of
these factors is of great importance, yet they may not be readily
determined except by actual experiment with various coils, owing to
the extreme complexity of the action which takes place in them and to
the difficulty of obtaining fundamental data as to the existing facts.
It may be stated, therefore, that the design of induction coils is
nearly always carried out by “cut-and-try” methods, bringing to bear,
of course, such scientific and practical knowledge as the experimenter
may possess.
[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 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
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.