the front edge of the piece 2 so that a slight rocking motion will be permitted on the “knife-edge” bearing thus afforded.  It is seen from the dotted lines that the magnetic circuit is almost a closed one.  The only gap is that between the lower end of the armature 3 and the front end of the core.  When the coil is energized, this gap is closed by the attraction of the armature.  As a result, the rearwardly projecting end of the armature 3 is raised and 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