POPULAR SCIENCE MONTHLY. 



this bar, having no attractive or repulsive force of its own, can 

 only obey the attractive action of the other, which is the only one 

 that exerts a force. 



In Fig. 4 M is a magnet bent into the form of a U, commonly 

 called a horseshoe magnet. The short bar set between the upper 

 ends is also a magnet, and is arranged so as to revolve around the 

 shaft s. From what has just been explained in connection with 

 Figs. 1 and 2 it will be understood that, with the poles as indi- 

 cated by the letters, there will be an attractive force set up be- 

 tween the top end of the straight bar and the P end of the horse- 

 shoe, and thus rotation will be produced in the direction of the 

 arrow. The rotation, however, will necessarily stop when the bar 

 reaches the position shown in Fig. 5, for then the attraction be- 

 tween the poles will resist further movement. If the straight bar 

 were not a magnet, but simply a piece of iron or steel, it is evident 

 tliat when in the position of Fig. 4 the attraction would be just 



as much toward the right as 

 toward the left, and if the 

 bar were placed accurately in 

 the central position it would 

 not swing in either direction. 

 It would be in the condition 

 called, in mechanics, unstable 

 equilibrium. In practice this 

 condition could not be very 

 w(dl realized, as it would be 

 difficult to set and retain the 

 l)ar in a position where the 

 attraction from both sides would be the same, therefore the rota- 

 tion would be in one direction or the other; but whichever way 

 the bar might move, it would only swing through one quarter of 

 a revolution, into the horizontal position of Fig. 5. 



If we reflect upon these actions we can see that if we could 

 destroy the magnetism of both parts before the straight bar reaches 

 the position of Fig. 5 it would be possible to obtain rotation through 

 a greater distance than one quarter of a turn, for then the head- 

 way acquired by the rotating part would cause it to continue its 

 motion. If, after the completion of one half of a revolution, we 

 could remagnetize both parts, we would then set iqi an attraction 

 between the lower end of the straight bar and the loft side of the 

 horseshoe, for then the polarity of the former would be the reverse 

 of that shown in Fig. 4 — that is, the lower end would be negative. 

 By means of this second attraction we would cause the bar to ro- 

 tate through the third quarter of the revolution, and if, just before 



F.g. 4. 



Fig. 5. 



Figs 4,5. — Diagkajis ii.listrating the Method 



OF OBTAINING Rotary iMoTION with MA(iNET.-l. 



