262 



GENERAL SCIENCE 



ELECTRIC BELL 



and the load of iron drops at once. Electromagnets are 

 also used in electric bells, in telegraph and telephone 

 instruments, and in electric power generators. 



182. The Common Electric Bell. The dry cell is 

 usually used for the operation of an electric bell, since a 



continuous current is not necessary. In 

 the illustration, when the push button, 



B, is pressed the electric circuit is closed, 

 and a current flows from the battery to 



C, thence through the coils of the elec- 

 tromagnet, over the closed contact A, 

 and out again at D. No sooner is this 

 current established than the electromag- 

 net E pulls over the armature a, and in 

 so doing breaks the contact at A. This 

 stops the current, and the magnet E at 



once loses its magnetism. The armature is then drawn 

 back against A by the spring S. As soon as the con- 

 tact is made at A the current again begins to flow 

 and the previous operation is repeated. The circuit is 

 thus automatically made and broken 

 at A, and the hammer h is made to 

 vibrate very rapidly, striking the bell 

 at each vibration, thus producing the PU * H BuTT N FOR 



ELECTRIC BELL 



ringing noise. 



183. The Telegraph. The electric telegraph and 

 the telegraphic code or alphabet were invented by S. F. B. 

 Morse in 1832. The first public exhibition of the instru- 

 ment was made in 1837 m New York City, when a 

 message was sent over a copper wire 1700 feet long. 

 The first commercial telegraph line was built in 1844, be- 

 tween Baltimore and Washington, by the aid of a $30,000 

 grant from Congress. On May 24, 1844, the inventor 



