magnetized as in an
ordinary magnet. Likewise, the core will be energized if a current be
sent from _5_ to _3_. Assuming that the two windings are of equal
resistance and number of turns, the effects so produced, when either
the coil _1_ or the coil _2_ is energized, will be equal. If the
battery be connected between the terminals _4_ and _5_ with the
positive pole, say, at _5_, then the current will proceed through the
winding _2_ and tend to generate magnetism in the core in the
direction of the arrow. After traversing the winding _2_, however, it
will then begin to traverse the other winding _1_ and will pass around
the core in the opposite direction throughout the length of that
winding. This will tend to set up magnetism in the core in the
opposite direction to that indicated by the arrow. Since the two
currents are equal and also the number of turns in each winding, it is
obvious that the two magnetizing influences will be exactly equal and
opposite and no magnetic effect will be produced. Such a winding, as
is shown in Fig. 96, where the two wires are laid on side by side, is
called a _parallel differential winding_.

Another way of winding magnets differentially is to put one winding on
one end of the core and the other winding on the other end of the core
and connect these so as to cause the currents through them to flow
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 adve