WHAT MAKES THE TROLLEY CAR GO. 319 



completing this last quarter, we were to remove all the magnetism 

 again, the headway would keep up the motion through the final 

 quarter of the revolution, thus completing one full turn. From 

 this it will be realized that if we could magnetize and demagnetize 

 the two parts twice in each revolution a continuous rotation could 

 be obtained. 



If the magnetizing and demagnetizing action were only applied 

 to the rotating part we would fail to keep up a continuous rota- 

 tion, for, as was shown in connection with Fig. 3, the action when 

 the straight bar reached the position of Fig. 5 would be the same 

 as if it were magnetized, owing to the fact that a magnet always 

 exerts an attraction upon a mass of iron. Suppose, however, that 

 we were to reverse the polarity of the rotating part just as it 

 reaches the position of Fig. 5, then there would be two poles of 

 the same polarity opposite each other, and, as shown in Fig. 2, 

 the force acting between them would be repulsive, and would push 

 the bar around in the direction of rotation. I^ot only would the 

 right-side pole of the horseshoe force the end of the bar away from 

 it, but the negative pole, on the left side, would attract this same 

 end, and thus a force would be exerted by the two poles of M to 

 keep up the rotation through the next half of a circle. On reach- 

 ing this last position the rotation would stop if the polarity of the 

 revolving bar were left unchanged, for then the poles facing each 

 other would be of opposite polarity. If, however, we again re- 

 versed the polarity, a rej)ulsion would be set up between the poles 

 facing each other, and thus a force would be exerted to continue 

 the rotation. Thus we see that if the polarity of the horseshoe 

 magnet is not disturbed it is necessary to reverse that of the rotat- 

 ing part to obtain a continuous motion, but if we change the mag- 

 netic conditions of both parts, then it is only necessary to magnetize 

 and demagnetize them alternately. 



From the foregoing it is seen that there are two ways in which 

 the force of magnetism could be utilized to keep up a continuous 

 rotation, and the question now is, Can either of them be made 

 available in practice? To this we answer that, by the aid of the 

 relations existing between electricity and magnetism, both can be 

 and are made available, as will be shown in the following para- 

 graphs : 



In Fig. 6 W represents a coil of wire provided with a cotton 

 covering, so that there may be no actual contact between the ad- 

 joining convolutions. If the ends p n of this coil are connected 

 with a source of electric energy, an electric current will flow 

 through it, and if a bar, as indicated by N P, of iron or steel is 

 placed within the coil it. will become magnetized. If the bar is 



