rse
conditions.

The most usual form of magnet construction employed in telephony is
shown in Fig. 98. On the core, which is of soft Norway iron, usually
cylindrical in form, are forced two washers of either fiber or hard
rubber. Fiber is ordinarily to be preferred because it is tougher and
less liable to breakage. Around the core, between the two heads, are
then wrapped several layers of paper or specially prepared cloth in
order that the wire forming the winding may be thoroughly insulated
from the core. One end of the wire is then passed through a hole in
one of the spool heads or washers, near the core, and the wire is then
wound on in layers. Sometimes a thickness of paper is placed around
each layer of wire in order to further guard against the breaking down
of the insulation between layers. When the last layer is wound on, the
end of the wire is passed out through a hole in the head, thus leaving
both ends projecting.

[Illustration: Fig. 98 Construction of Electromagnet]

Magnet Wire. The wire used in winding magnets is, of course, an
important part of the electromagnet. It is always necessary that the
adjacent turns of the wire be insulated from each other so that the
current shall be forced to pass around the core through all the length
of wire in each turn rather than allowing it to take the shorter and
easier path from one turn to the next, as would be the case if the
turns were not insulated. For this purpose the wire is usually covered
with a coating of some insulating material. There are, however,
methods of winding magnet coils with bare wire and taking care of the
insulation between the turns in another way, as will be pointed out.

Insulated wire for the purpose of winding magnet coils is termed
_magnet wire_. Copper is the material almost universally employed for
the conductor. Its high conductivity, great ductility, and low cost
are the factors which make it superior to all other metals. However,
in special cases, where exceedingly high conductivity is required with
a limited winding space, silver wire is sometimes employed, and on the
other hand, where very high resistance is desired within a limited
winding space, either iron or German silver or some other
high-resistance alloy is used.

_Wire Gauges_. Wire for electrical purposes is drawn to a number of
different standard gauges. Each of the so-called wire gauges consists
of a series of graded sizes of wire, ranging from approximately
one-h