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. [Illustration: Fig. 119.] _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. [Illustration: Fig. 120.] =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. [Illustration: Fig. 121.] [Illustration: Fig. 122.] [Illustration: Fig. 123.] [Illustration: Fig. 124.] =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. CHAPTER XVI. THE TELEPHONE, AND HOW IT TRANSMITS SPEECH. =119. The Telephone= is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. The part spoken to is called the _transmitter_, and the part which gives sound out again is called the _receiver_. Sound itself does not pass over the line. While the same apparatus can be used for both transmitter and receiver, they are generally different in construction to get the best results. [Illustration: Fig. 125.] [Illustration: Fig. 126.] [Illustration: Fig. 127.] =120. The Bell or Magneto-transmitter= generates its own current, and is, strictly speaking, a dynamo that is run by the voice. It depends upon induction for its action. [Illustration: Fig. 128.]