ELECTRICITY. 



79 



one wire sets up induced currents in neighboring wires. One of the 

 principal applications made of these induced currents is in the induc- 

 tion coil, or Ruhmkorf coil, shown diagrammatically in Fig. 30. It 

 consists of a hollow cylinder covered with a coil of insulated and 

 relatively coarse wire, P, connected with a battery, B. Over this 

 first or primary coil is wound another, the secondary coil, S, com- 

 posed of much longer and finer wire. 



FIG. 30. 



Induction coil. 



While a current passes through the primary wire nothing is noticed 

 in the secondary wire, but the instant the current is closed an instan- 

 taneous induced current is set up in the secondary wire in one direc- 

 tion, and on opening the circuit in the opposite direction. It follows 

 that it requires a rapid closure and opening of the circuit to generate 

 electricity in the secondary coil. The opening and closing of the 

 current are accomplished by the following self-acting arrangement. 

 In the hollow portion of the primary coil are placed bars of soft iron, 

 which are magnetized whenever a current passes through the coil. 

 Near one terminal of the iron bars is a movable metallic hammer, H, 

 held by a spring in such a position that the current is closed by it. 

 When the bars are magnetized, they raise the hammer, thereby open- 

 ing the circuit ; the cessation of the current causes the iron to be de- 

 magnetized and the hammer falls to its original position, closing the 

 circuit, and this action continues as long as the current is allowed 

 to flow. This arrangement, known as a " make and break " con- 

 trivance, generates, through the very rapid opening and closing of 

 the primary circuit, in the secondary coil that current which is known 

 as secondary, induced, interrupted, or/aradic current. 



The object of the induction coil is to generate from a battery current of low 

 E. M. F. induced currents of very high E. M. F. The effectiveness of the 



