500 MAGNETISM AND ELECTRICITY . 



transmitted by wires and trolley through the controller 

 to the motor under the street car. The motorman, by 

 means of the controller, is able to turn the current into the 

 motor or to shut it off. When the current is turned on, 

 the motor revolves; by gearings the motion is imparted to 

 the wheels and the car moves. Thus the electricity gen- 

 erated by the dynamos in power houses, wherever they 

 may be, not only lights our homes and streets, but also en- 

 ables the little motors in our homes, the powerful motors 

 on street cars, and the giant motors of our factories to do 

 all kinds of work for us. 



Theory of Electricity. A great deal is known about how 

 electricity acts and what it does, but as yet little is known 

 about what it really is. Recent experiments indicate that 

 the atoms of matter (page 51) contain electricity, and 

 that the negative electricity in them exists in the form 

 of exceedingly minute particles called electrons. There 

 are hundreds of these electrons in each atom, and they are 

 held there probably by the attraction of a positive charge 

 of electricity at the center of the atom. If the positive and 

 negative charges in the atoms of a body are equal, the body 

 is unelectrified. 



If, however, the electrons are in any way joggled off and 

 accumulated, a negative charge of electricity develops 

 where this accumulation takes place. As the electrons are 

 all negative, they repel one another and tend to move away 

 from the point where they have accumulated to places 

 where the accumulation is not so great. This is what hap- 

 pened in Experiment 156, when the electrical machine was 

 used. An electric current is supposed to be a stream of 

 these electrons. 



