Audible Signals

Telegraph Sounder. The earliest electric signal was an audible one, being the telegraph sounder, or the Morse register considered apart from its registering function. Each telegraph sounder serves as an audible electric signal and is capable of signifying more than that the call is being made. Such a signal is operated by the making and breaking of current from a battery. An arrangement of this kind is shown in Fig. 15, in which pre ure upon the key causes the current from the battery to energize the sounder and give one sharp audible rap of the lever upon the striking post. Vibrating Bell . The vibrating bell, so widely used as a door bell, is a device consequent to the telegraph. Its action is to give a series of blows on its gong when its key or push button closes the battery circuit. At the risk of describing a trite though not trivial thing, it may be said that when the contact 1 of Fig. 16 is closed, current from the battery energizes the armature 2 , causing the latter to strike a blow on the gong and to break the line circuit as well, by opening the contact back of the armature. So de-energized, the armature falls back and the cycle is repeated until the button contact is released. A comparison of this action with that of the polarized ringer (to be described later) will be found of interest. IMG:384091999325875926_fig017_t.gif.png:Illustration_ Fig. 17. Elemental Magneto-Generator Fig. 17. Elemental Magneto-Generator View full size illustration. 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 transmi ion was supplied by the muscular ti ues of the user—a case of working one's pa age. IMG:384091999325875926_fig018_t.gif.png:Illustration_ Fig. 18. Extension of a Permanent Magnet Fig. 18. Extension of a Permanent Magnet View full size illustration. 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. IMG:384091999325875926_fig019_t.gif.png:Illustration_ Fig. 19. Extension of a Permanent Magnet Fig. 19. Extension of a Permanent Magnet View full size illustration. "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. 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. IMG:384091999325875926_fig020_t.gif.png:Illustration_ Fig. 20. Electromagnet Fig. 20. Electromagnet View full size illustration. IMG:384091999325875926_fig021_t.gif.png:Illustration_ Fig. 21. Polarized Ringer Fig. 21. Polarized Ringer View full size illustration. 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 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. 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 discu ion 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.