around the core in opposite directions. Such a construction is shown
in Fig. 97 and is called a _tandem differential winding_. The tandem
arrangement, while often good enough for practical purposes, cannot
result in the complete neutralization of magnetic effect. This is true
because of the leakage of some of the lines of force from intermediate
points in the length of the core through the air, resulting in some of
the lines passing through more of the turns of one coil than of the
other. Complete neutralization can only be attained by first twisting
the two wires together with a uniform lay and then winding them
simultaneously on the core.

[Illustration: Fig. 97. Tandem Differential Electromagnet]

Mechanical Details. We will now consider the actual mechanical
construction of the electromagnet. This is a very important feature of
telephone work, because, not only must the proper electrical and
magnetic effects be produced, but also the whole structure of the
magnet must be such that it will not easily get out of order and not
be affected by moisture, heat, careless handling, or other adverse
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