Chapter XV: The Electric Bell and Some of Its Uses

=115. Automatic Current Interrupters= are used on most common bells, as well as on induction coils, etc. (See § 104.) Fig. 115 shows a simple form of interrupter. The wire 1, from a cell D C, is joined to an iron strip I a short distance from its end. The other wire from D C passes to one end of the electromagnet coil H. The remaining end of H is placed in contact with I as shown, completing the circuit. As soon as the current passes, I is pulled down and away from the upper wire 2, breaking the circuit. I, being held by its left-hand end firmly in the hand, immediately springs back to its former position, closing the circuit again. This action is repeated, the rapidity of the vibrations depending somewhat upon the position of the wires on I. In regular instruments a platinum point is used where the circuit is broken; this stands the sparking when the armature vibrates.

=116. Electric Bells= may be illustrated by referring to Fig. 116, which shows a circuit similar to that described in § 115, but which also contains a key K, in the circuit. This allows the circuit to be opened and closed at a distance from the vibrating armature. The circuit must not be broken at two places at the same time, so wires should touch at the end of I before pressing K. Upon pressing K the armature I will vibrate rapidly. By placing a small bell near the end of the vibrating armature, so that it will be struck by I at each vibration, we should have a simple electric bell. This form of electric bell is called a _trembling_ bell, on account of its vibrating armature.

Fig. 117 shows a form of trembling bell with cover removed. Fig. 118 shows a _single-stroke_ bell, used for fire-alarms and other signal work. In this the armature is attracted but once each time the current passes. As many taps of the bell can be given as desired by pressing the push-button. Fig. 119 shows a gong for railway crossings, signals, etc. Fig. 120 shows a circuit including cell, push-button, and bell, with extra wire for lengthening the line.

_Electro-Mechanical Gongs_ are used to give loud signals for special purposes. The mechanical device is started by the electric current when the armature of the electromagnet is attracted. Springs, weights, etc., are used as the power. Fig. 121 shows a small bell of this kind.

=117. Magneto Testing Bells=, Fig. 122, are really small hand-power dynamos. The armature is made to revolve between the poles of strong permanent magnets, and it is so wound that it gives a current with a large E. M. F., so that it can ring through the large resistance of a long line to test it.

_Magneto Signal Bells_, Fig. 123, are used as generator and bell in connection with telephones. The generator, used to ring a bell at a distant station, stands at the bottom of the box. The bell is fastened to the lid, and receives current from a distant bell.

=118. Electric Buzzers= have the same general construction as electric bells; in fact, you will have a buzzer by removing the bell from an ordinary electric bell. Buzzers are used in places where the loud sound of a bell would be objectionable. Fig. 124 shows the usual form of buzzers, the cover being removed.