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EXPERIMENTAL GENERAL SCIENCE 



of soft iron, called a core, the magnetic force of both coil and 

 iron are greatly increased, and the effects continue as long as 

 the current is flowing. When the current ceases, the iron is 

 no longer magnetic. Magnets made in this way are called 

 electromagnets. The most common form is of the familiar 

 horseshoe shape. Such magnets are an essential part of 

 our electric bells, annunciators, telephones, and telegraph 

 instruments. Electric cranes, consisting of powerful electro- 

 magnets, are used for lifting heavy pieces of steel and iron 

 in mills and factories. The electric bell is made to ring by 



FIG. 76. Diagram of an 



electroscope. (Tower, Smith 

 and Turton.) 



FIG. 77. An electric bell and its circuit- 

 A, Armature; M, electromagnet. (Tower, 

 Smith and Turton.) 



a device that alternately makes and breaks the current flowing 

 through an electromagnet of horseshoe form. A piece of soft 

 iron, called the armature, opposite the poles of the magnet 

 is attached to the hammer of the bell, and, when not in action, 

 a spring holds it away from the magnet and in contact with the 

 wire from one pole of the cell or battery. When the circuit is 

 closed, the current of electricity passes by way of the armature 

 through the coils of wire in the horseshoe making it an electro- 

 magnet, and this at once attracts the armature and causes 

 a stroke of the bell. The motion of the armature, however, 



