Electric Generators. 71 



frame and mount the soft iron with the wire 

 on it (which in this shape is an electromagnet) 

 on a revolving arm and so set it on the arm 

 that its ends will come close to, but not touch, 

 the poles of the permanent magnet. Now re- 

 volve the arm, and every time the electromag- 

 net or keeper approaches the permanent mag- 

 net a current of one polarity will be momen- 

 tarily developed in the wire of the electromag- 

 net, which is moving. When it is opposite 

 the poles, it has reached the maximum charge 

 and, new, as it passes on it discharges and a 

 current of the opposite polarity is developed in 

 the wire. The more rapidly we revolve the 

 arm the more voltage (electrical pressure) the 

 current it develops will have. 



It will be plain to all that we might make 

 the electromagnet stationary and revolve the 

 permanent magnet and get the same result. If 

 the permanent magnet were strong enough 

 and the electromagnet the right size as to iron, 

 windings, etc., and we revolve the arm with 

 sufficient rapidity, we could get an alternating 

 current of electricity that would produce an 

 electric li^lit. I have not and cannot here give 

 you the construction of a modern alternating- 

 current dynamo. I have simply described tin- 

 simplest form of dynamo, ;m<l all of them oper- 

 ate upon the fundamental principle of a per- 

 it magnetic field ami an electromagnet, 

 moving in a certain relation to each other. 



