HISTORY OF SCIENCE. 



current was brought near a mobile one, as for example the wire B B 

 represented in the figure as held in the hands, certain attractions and 

 repulsions manifested themselves. The laws of these forces may be 

 expressed thus: Parallel currents attract each other if they are flowing 

 in the same direction, and repel each other if they are flowing in opposite 

 directions. The current free to move will always tend to place itself 

 parallel to and in the same direction with the fixed current. 



Ampere was able to sum up all the observed facts of the mutual 

 actions of magnets and currents by a theory of magnetism which sup- 

 poses that round each molecule of iron and steel an electric current 

 is continually circulating. In the ordinary condition of a piece of iron 

 or steel these currents circulate in every possible direction, and the 

 magnetization consists in giving to the molecular currents a deter- 



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/ ^^>v^ 



FIG. 276. 



FIG. 277. 



minate direction, which, in the case of steel, they permanently retain. 

 This being admitted, all the rest can be deduced by the elementary 

 laws of the mutual actions of currents (electro-dynamics). 



The resultant action of a series of currents circulating in one direc- 

 tion about the particles of a magnet would be equivalent to that of a 

 single current moving in the same direction about the circumference 

 of the magnet. If A, B, etc., Fig. 277, represent sections of the mole- 

 cular filaments which collectively form a magnetized bar, it will be 

 seen that the effects of those currents on external bodies will be neu- 

 tralized as regards all the inner currents, because everywhere these are 

 contiguous to similar currents in the opposite direction. The outer part 

 of the bar will, however, have uncompensated currents, and the aggre- 

 gate of these would be equivalent to a current circulating about the 

 circumference of the bar. Fig. 278 represents the direction of the 

 resultant currents in a bar magnet N being the north, and s the south 

 pole. 



