140 MAGNETS AND ELECTRICITY 



down, and the circuit is thus closed, the core is magnetized, 

 and draws the sounder to the lower stop. This produces 

 a " click." When the key is released, the sounder springs 

 back. 



The current is usually supplied by a "gravity" battery. Only one 

 wire is needed, for the return circuit is through the earth (cf. 146). 

 There is no interference between these earth circuits, in spite of the 

 large number of messages passing at the same time. 



In the original telegraph, as made by Morse in 1832, a moving strip 

 of paper was placed under the sounder. If the circuit was kept closed 

 a very short time, a dot was made upon the paper; if for a longer time, 

 a dash was the result. Combinations of dots and dashes made up the 

 alphabet. The "printing telegraph" is rarely used now, for messages 

 are taken much more rapidly by ear. 



The telegraph code is as follows : 



a- k u -- 1 



b / v 2 



c - - - m - w 3 



d n x 4 



e - o - - y - - - - 5 



/ --- P 6 



g - q & 



h r - - - , 8 



t r - S --- ? 9 



j t - . 



156. The Electric Bell. The electric bell (Fig. 130) 

 has a push-button for closing the circuit and thus operat- 

 ing the electromagnet. * The hammer of the bell and the 

 armature are attached to a spring. When the electro- 

 magnet attracts the armature, the hammer strikes the 

 bell. But at the same instant the circuit is broken, and 

 the spring throws the armature back against the stop. 

 The circuit is thus closed once more, and the armature is 

 again attracted. In this way the hammer is made to give 

 the bell a rapid succession of blows. 



