A permanent magnet so related to iron forms part of a polarized ringer.  So does an electromagnet composed of windings and iron cores.  Fig. 20 reminds us of the law of electromagnets.  If current flows from the plus towards the minus side, with the windings as drawn, polarities will be induced as marked.

[Illustration:  Fig. 20.  Electromagnet]

[Illustration:  Fig. 21.  Polarized Ringer]

If, now, such an electromagnet, a permanent magnet, and a pivoted armature be related to a pair of gongs as shown in Fig. 21, a polarized ringer results.  It should be noted that a permanent magnet has both its poles presented (though one of the poles is not actually attached) to two parts of the iron of the electro-magnet.  The result is that the ends of the armature are of south polarity and those of the core are of north polarity.  All the markings of Fig. 21 relate to the polarity produced by the permanent magnet.  If, now, a current flow in the ringer winding from plus to minus, obviously the right-hand pole will be additively magnetized, the current tending to produce north magnetism there; also the left-hand pole will be subtractively magnetized, the current tending to produce south magnetism there.  If the current be of a certain strength, relative to the certain ringer under study, magnetism in the left pole will be neutralized and that in the right pole doubled.  Hence the armature will be attracted more by the right pole than by the left and will strike the right-hand gong.  A reversal of current produces an opposite action, the left-hand gong being struck.  The current ceasing, the armature remains where last thrown.

[Illustration:  OPERATOR’S EQUIPMENT Clement Automanual System.]

It is important to note that the strength of action depends upon the strength of the current up to a certain point only.  That depends upon the strength of the permanent magnet.  Whenever the current is great enough just to neutralize the normal magnetism of one pole and to double that of the other, no increase in current will cause the device to ring any louder.  This makes obvious the importance of a proper permanent magnetism and displays the fallacy of some effort to increase the output of various devices depending upon these principles.  This discussion of magneto-electric signaling is introduced here because of a belief in its being fundamental.  Chapter VIII treats of such a signaling in further detail.

Telephone Receiver. The telephone receiver itself serves a useful purpose as an audible signal.  An interrupted or alternating current of proper frequency and amount will produce in it a musical tone which can be heard throughout a large room.  This fact enables a telephone central office to signal a subscriber who has left his receiver off the switch hook, so that normal conditions may be restored.