1855.] on Magnetic Repulsimi. ' 15 



former experiment. The reason was deemed manifest : the state of 

 excitement which causes a to be attracted by N causes it to be 

 repelled by S ; both poles, therefore, act in unison, and a deflection 

 of greater energy is produced. 3. The pole S being removed to the 

 position S', the deflection was also found to be about twice as forcible 

 as when the single pole N was employed. Here also the reason is 

 plain : the two ends, a and b, of the bismuth bar, are in diiierent 

 states of excitement ; the end a is attracted by a north pole, the 

 end b is attracted by a south pole : both poles act therefore as a 

 mechanical couple upon the bar, and produce the deflection 

 observed. 4. The pole S' was replaced by a north pole of the 

 same strength, thus bringing two poles of the same name to bear 

 upon the two ends of the bar : there was no deflection by this 

 arrangement ; it is manifest that N s attraction for the end a was 

 nullified by the repulsion of the end 6 by a like pole ; the experi- 

 ment thus furnishes an additional proof of the polar condition of a b. 

 5. We have supposed the pole S to be removed into the position 

 S' ; but permitting the pole S to remain, and introducing another 

 pole (a south one) at S', a greater action than that produced with 

 two magnets was obtained. 6. Finally, adding another north pole at 

 N', and allowing four magnets to operate upon the bismuth bar 

 simultaneously, a maximum action was obtained, and the bar was 

 attracted and repelled with the greatest promptness and decision. 

 I7i all these cases where an iron bar was substituted for the bismuth 

 bar a b, a deflection precisely the opposite to that exhibited by 

 a b wa^ produced. A branch of the current by which the bar of 

 bismuth was surrounded could be suffered to circulate round a bar 

 of iron, suspended freely in an adjacent helix ; when the forces 

 acting upon the iron were the same as those acting upon the bismuth, 

 the bars were always deflected in opposite directions. 



The question of dia-magnetic polarity was next submitted to a 

 test which brought it under the dominion of the principles of me- 

 chanics. A mass of iron was chosen for the moveable magnetic 

 pole, of such a shape that the diminution of the force emanating 

 from the pole, as the distance was augmented, was very slow ; or in 

 other words, the field of force was very uniform. Let the space in 

 front of the pole P, (Fig. 2.) be such ^ 



a field. A normal bar of bismuth, 

 a b, was attached to the end of a lever 

 transverse to the length of the latter, 

 and counterpoised by a weight at the 

 other extremity : the system was then 

 suspended from its centre of gravity 

 ^, so that the beam and bar swung 

 horizontally. Supposing the bar to oc- 

 cupy the position shown in the figure, 

 then if the force acting upon it be purely repulsive — that is to say, 

 if the dia-magnetic force be unpolar — it is evident that the tendency 



