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phone A. If, therefore, a person talks against the diaphragm e so that the sound waves of his voice cause it to vibrate, the diaphragm i will also vibrate in the same manner; and as the conditions at B are simply those at A reversed, the vibration of the diaphragm i will reproduce at B the sounds delivered at A.

In the transmission of speech, the telephone instrument into which the sound waves of the voice are delivered is called the transmitter, and that in which the sound waves are reproduced is called the receiver. In the description just given of Fig. 1, the telephone A has been referred to as the transmitter, and the telephone B as the receiver. Both these instruments in Fig. I are constructed alike, so it makes no difference in that case which one is used as the transmitter and which one as the receiver. In practice, however, the receiver alone follows the design of the magnet telephones in Fig. I, the transmitter being constructed differently in order to increase the range of transmission.

Different Forms of Receivers are used to meet different conditions of service. The original single-pole receiver, of which those shown in Fig. I are a type, is still used for ordinary work over short distances. A commercial receiver of this form is shown in Fig. 2, A showing the case and interior construction with a side view of the magnet, and B an end view of the magnet. The magnet is formed of four picces c, l, r, n, of hard magnet

ized steel, two of which are placed on cach side of the flattened iron end projections u and v, and all are bolted together at each end. The end projections h and I are of cylindrical shape and serve, the one for holding a wooden or fiber spool which carries the coil of insulated wire s, and the

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Fig. 2.-Single-Pole Receiver and Details of the Magnet

Used in It

other for mounting the magnet to the surrounding hard-rubber case d. The coil s is usually wound with No. 38 B. & S. gage silk-covered copper wire to a resistance of 75 ohms, and its terminals are connected to the binding posts m and o by No. 18 or 20 B. & S. gage cotton-covered copper wire which is soldered to them. The magnet should have sufficient power to hold a weight

of 16 ounces. The diaphragm i is of soft iron 0.01 inch thick, is perfectly flat, and is held 1-32 inch distant from the end of the magnet by means of the hard-rubber cap k.

The Double-Pole Receiver for several years past has been gradually replacing the single pole receiver on account of its greater sensitiveness, and is now used entirely in long-distance transmission. Fig. 3 shows a standard receiver of this form. As its name implies, the magnet has two poles presented to the diaphragm; it is of horseshoe shape, the 'b od pole tips s and s being bolted Fig. 3.—The Double

Pole Receiver to the legs n and n of the magnet and carry the coils 0 and 0, which are mounted upon brass spools. The coils are con

Fig. 4.—Method of Winding the Magnets for a Double

Pole Receiver

nected in series and are wound in opposite directions as in Fig. 4, with No. 36 ur 38 B. & S.

gage silk-covered copper wire to a resistance depending upon the conditions of service. For local battery work, such as considered in this book, the resistance of the receiver is usually from 100 to 125 ohms (see Appendix) divided equally between the two coils. Cotton-covered copper wires are soldered to and connect the magnet windings with the inner parts of the binding posts c and c. The diaphragm is of ferrotype metal (a species of soft iron) No. 31 B. W. G., 0.01 inch thick, 21 inches in diameter over all, with a free diameter of about 2 inches, and a diameter of about 1 inch exposed to the sound waves. It is perfectly flat, and is varnished as a protection from corrosion. The .magnet is turned upon the screw a, which holds it to the case, until the pole faces are 32 inch distant from the diaphragm d. If there be a greater distance than this, the magnetic effect will be too weak, and, if less, the diaphragm is liable to stick to the pole faces. The magnet should be capable of exerting a tractive effort of 1.13 pounds. Between the legs of the magnet a lead weight w is cast to increase the weight of the receiver and thus insure its holding down the hook switch in its proper position.

The inclosing case is of hard rubber and comprises three parts: the body b, the tailpiece t, and the diaphragm cap m. Although the cap m can be unscrewed, permitting the diaphragm to be taken out and examined, it is impossible to take the receiver further apart without unsolder

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Fig. 5.-A Double-Pole Receiver with Interior Binding Posts for the Receiver Cord

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