Action of Magnets on Copper and good Conductors. 467 



pelled, and thus I believe all the actions up to this condition 

 of things is accounted for. 



2336. Then comes the revulsion which occurs upon the 

 cessation of the electric current, and the falling of the mag- 

 netism in the core. According to the law of magneto-electric 

 induction, the disappearance of the magnetic force will induce 

 brief currents in the copper bar (28.), but in the contrary 

 direction to those induced in the first instance; and there- 

 fore the virtual magnetic pole belonging to the copper for 

 the moment, which is nearest the north end of the electro- 

 magnet, will be a south pole; and that which is furthest from 

 the same pole of the magnet will be a north pole. Hence 

 will arise an exertion of force on the bar tending to turn it 

 round its centre of suspension in the contrary direction to that 

 which occurred before, and hence the apparent revulsion ; for 

 the angle nearest the magnetic pole will recede from it, the 

 broad face (2323.) or length (231.5.) of the bar will come round 

 and face towards the magnet, and an action the reverse in 

 every respect of the first, action will take place, except that 

 whereas the motion was then only a few degrees, now it may 

 extend to two or three revolutions. 



2337. The cause of this difference is very obvious. In the 

 first instance, the bar of copper was moving under influences 

 powerfully tending to retard and stop it (2329.) ; in the second 

 case these influences are gone, and the bar revolves freely with 

 a force proportionate to the power exerted by the magnet upon 

 the currents induced by its own action. 



2338. Even when the copper is of such form as not to give 

 the oblique resultant of magnetic action from the currents in- 

 duced in it, when, for instance, it is a cube or a sphere, still 

 the effect of the action described above is evident (2325.). 

 When a plate of copper about three-fourths of an inch in 

 thickness, and weighing two pounds, was sustained upon some 

 loose blocks of wood and placed about 0*1 of an inch from the 

 face of the magnetic pole, it was repelled and held off a certain 

 distance upon the making and continuing of electric contact 

 at the battery ; and when the battery current was stopped, it 

 returned towards the pole; but the return was much more 

 powerful than that due to gravity alone (as was ascertained by 

 an experiment), the plate being at that moment actually at- 

 tracted^ as well as tending by gravitation towards the magnet, 

 so that it gave a strong tap against it. 



2339. Such is, I believe, the explanation of the peculiar 

 phamomena presented by copper in the magnetic field ; and 

 the reason why they appear with this metal and not with bis- 

 muth or heavy glass, is almost certainly to be found in its high 



