Electric Currents. 57 



dial and a deflecting needle or pointer). Now 

 if we touch the key and close the circuit in A, 

 the needle of the galvanometer in B will swing 

 in one direction from zero on the dial ; and if 

 we release the key, breaking the circuit in A, 

 the needle will swing back in the opposite 

 direction. In neither case will the needle stay 

 deflected, but will at once return to zero. 



This shows that when the battery current 

 was allowed to complete its circuit through 

 wire A by closing its key, an electrical action 

 instantly felt in wire B, although there 

 was no material connection between them other 

 than the air, which is a non-conductor. 



The current in the second circuit is called 

 an induced current. Why this current? Ac- 

 cording to one theory, when we close the pri- 

 mary circuit the surrounding ether is thrown 

 into a peculiar state of strain that we will call 

 magnetic or electrical lines of force. When 

 the ether wave strikes the second wire there is 

 a molecular movement from a state of rest to a 

 state of static strain. During the time that 

 the molecules are moving from the normal to 

 the strained position in sympathy with the 

 ether we have the condition of a dynamic cur- 

 rent, \vhii-h lasts only a moment. This state of 

 strain <-,ntinues till the circuit is opened 

 (breaking the win-line), when all the elec- 

 trie.il lines of force vanish and the molecular 

 strain of the second wire is relieved, and we 



