this raises the spring
_4_ and causes it to break the normally existing contact with the spring
_5_ and to establish another contact with the spring _6_. Thus the
energy developed within the coil of the magnet is made to move certain
parts which in turn operate the switching devices to produce changes in
electrical circuits. These relays and other adaptations of the
electromagnet will be discussed more fully later on.

[Illustration: Fig. 95. Electromagnet of Relay]

There are almost numberless forms of electromagnets, but we have
illustrated here examples of the principal types employed in
telephony, and the modifications of these types will be readily
understood in view of the general principles laid down.

Direction of Armature Motion. It may be said in general that the
armature of an electromagnet always moves or tends to move, when the
coil is energized, in such a way as to reduce the reluctance of the
magnetic circuit through the coil. Thus, in all of the forms of
electromagnets discussed, the armature, when attracted, moves in such a
direction as to shorten the air gap and to introduce the iron of the
armature as much as possible into the path of the magnetic lines, thus
reducing the reluctance. In the case of a solenoid type of
electromagnet, or the coil and plunger type, which is a better name than
solenoid, the coil, when energized, acts in effect to suck the iron core
or plunger within itself so as to include more and more of the iron
within the most densely occupied portion of the magnetic circuit.

[Illustration: Fig. 96. Parallel Differential Electromagnet]

Differential Electromagnet. Frequently in telephony, the
electromagnets are provided with more than one winding. One purpose of
the double-wound electromagnet is to produce the so-called
differential action between the two windings, _i.e._, making one of
the windings develop magnetization in the opposite direction from that
of the other, so that the two will neutralize each other, or at least
exert different and opposite influences. The principle of the
differential electromagnet may be illustrated in connection with Fig.
96. Here two wires _1_ and _2_ are shown wrapped in the same direction
about an iron core, the ends of the wire being joined together at _3_.
Obviously, if one of these windings only is employed and a current
sent through it, as by connecting the terminals of a battery with the
points _4_ and _3_, for instance, the core will be