298 



Professor J. A. Fleming 



[March 6, 



Fig. 1. 



eddy currents, and it screens the secondary circuit because the 

 indiictivt; action of these eddy currents on the side remote from the 

 magnet is exactly equal and opposite to that of the primary circuit 

 on the secondary coil. 



§ 3. Connecting together these elementary facts, we are easily able 

 to explain another well-known fact. If a continuous current is sent 



through the coils of a large electro- 

 magnet, and we magnetise its iron core 

 very powerfully, it is found to be im- 

 possible to strike the pole of the magnet 

 a sharp blow by means of a sheet of 

 copper. Holding a sheet of copper over 

 such a magnetic j)ole, and energising 

 the magnet, the hand holding the 

 copper sheet feels a repulsive action at 

 the moment when the current is put 

 on and an attractive action when it is 

 cut off. If we try to slap the magnet 

 sharply with the copper sheet, it is 

 found that this repulsive force prevents 

 anything like such a sharp blow being 

 given to the pole when the current is 

 on, as can be given when the current is 

 off. Moreover, when a very powerful 

 electro-magnet is employed, it is found 

 that a disc of copper let fall over the 

 pole does not fall down sharply and 

 quickly on to it when the current is 

 flowing through the coils of the mag- 

 Copper plate interposed between ^^t, but settles down softly and slowly, 

 LrSdTngX":e'Sa°y as if falling tWgh some sticky fluid, 

 from induction. We know that the correct explanation 



of these facts is to be found in the 

 statem(nt that the motion of the conductor towards the magnetic 

 pole causes eddy electric currents to be generated in it by electro- 

 magnetic induction, and that these being in the opposite direction 

 to the energising current of the magnet, cause a repulsive force to 

 exist between the inducing and secondary circuits, which creates the 

 apparent resistance we feel. 



In order to exhibit the stress brought into existence between an 

 electro-magnet and a metal sheet held near it when induced currents 

 are set up in the disc, we have arranged the following experiment : — 

 Over the pole of a powerful electro-magnet we have balanced a small 

 disc of copper, the size of a penny, carried on one end of a deli- 

 cately balanced bar. A mirror attached to that bar serves to reflect 

 on to the screen a ray of light indicating the smallest motion of the 

 copper piece. On magnetising the magnet the copper is suddenly 

 repelled, but comes to rest again immediately in its initial position. 



