e transmission
by induction from one of these circuits to the other. This is a
function that in telephony is scarcely of less importance than the
purely transforming function.

_Design._ Induction coils, as employed in telephony, may be divided
into two general types: first, those having an open magnetic circuit;
and, second, those having a closed magnetic circuit. In the design of
either type it is important that the core should be thoroughly
laminated, and this is done usually by forming it of a bundle of soft
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 w