Magneto-Bell. The magneto-bell came into wide use with the spread of telephone service.  Its two fundamental parts are an alternating-current generator and a polarized bell-ringing device.  Each had its counterpart long before the invention of the telephone, though made familiar by the latter.  The alternating-current generator of the magneto-bell consists of a rotatable armature composed of a coil of insulated wire and usually a core of soft iron, its rotation taking place in a magnetic field.  This field is usually provided by a permanent magnet, hence the name “magneto-generator.”  The purist in terms may well say, however, that every form whatever of the dynamo-electric generator is a magneto-generator, as magnetism is one link in every such conversion of mechanical power into electricity.  The terms magneto-electric, magneto-generator, etc., involving the term “magneto,” have come to imply the presence of permanently magnetized steel as an element of the construction.

In its early form, the magneto-generator consisted of the arrangement shown in Fig. 17, wherein a permanent magnet can rotate on an axis before an electromagnet having soft iron cores and a winding.  Reversals of magnetism produce current in alternately reversing half-cycles, one complete rotation of the magnet producing one such cycle.  Obviously the result would be the same if the magnet were stationary and the coils should rotate, which is the construction of more modern devices.  The turning of the crank of a magneto-bell rotates the armature in the magnetic field by some form of gearing at a rate usually of the order of twenty turns per second, producing an alternating current of that frequency.  This current is caused by an effective electromotive force which may be as great as 100 volts, produced immediately by the energy of the user.  In an equipment using a magneto-telephone as both receiver and transmitter and a magneto-bell as its signal-sending machine, as was usual in 1877, it is interesting to note that the entire motive power for signals and speech transmission was supplied by the muscular tissues of the user—­a case of working one’s passage.

[Illustration:  Fig. 18.  Extension of a Permanent Magnet]

The alternating current from the generator is received and converted into sound by means of the polarized ringer, a device which is interesting as depending upon several of the electrical, mechanical, and magnetic actions which are the foundations of telephone engineering.

[Illustration:  Fig. 19.  Extension of a Permanent Magnet]

“Why the ringer rings” may be gathered from a study of Figs. 18 to 21.  A permanent magnet will impart temporary magnetism to pieces of iron near it.  In Fig. 18 two pieces of iron are so energized.  The ends of these pieces which are nearest to the permanent magnet 1 are of the opposite polarity to the end they approach, the free ends being of opposite polarity.  In the figure, these free ends are marked N, meaning they are of a polarity to point north if free to point at all.  English-speaking persons call this north polarity.  Similarly, as in Fig. 19, any arrangement of iron near a permanent magnet always will have free poles of the same polarity as the end of the permanent magnet nearest them.