300 



Professor- J. A. Fleming 



[March 6, 



or the force which the ring experiences, is at any instant proportional 

 to the product of the strength of the magnetic field in which the 

 ring is immersed, and to the strength of the induced current created 

 in it. If we multiply together the numerical values of the ordinates 

 of these two curves at any and every point on the horizontal line, 

 and set up a new ordinate at that point representing this product, the 

 extremities of these last ordinates define a curve, which is a curve 

 representing the /orce acting on the secondary circuit, and it is, as 

 you see from the diagram, Fig. 3, a wavy curve having a wave length 

 equal to half that of the first two curves. Moreover, the whole area 

 inclosed between the outline of this force curve and the horizontal 

 line represents to a certain scale the time integral of that force, or 



Fig. 3. 



AttTactive InipuLtt* 







Diagram showing the equality of the attractive and repulsive impulses in a 

 non-inductive circuit when held in an alternating magnetic field. 



the impulse acting on the secondary circuit, and the theory shows us 

 that, under the assumptions made, the secondary circuit so acted upon 

 experiences in each period of the current four impulses, two positive 

 or repulsive, and two negative or attractive. Hence, it comes to this, 

 that such an ideal conducting circuit held in front of an alter- 

 nating electro-magnet should experience a rapid alternate series of 

 equal pushes and pulls, or of little impulses to and from the magnet. 

 These equal and opposite impulses in quick succession would 

 neutralise one another, and our supposed circuit would not, on the 

 whole, be subject to any resultant force. 



§ 5. Quite otherwise is it, however, when we present a real 

 conducting circuit to the pole of an electro-magnet traversed by a 

 powerful alternating current. We find that under the actual circum- 

 stances there is a powerful repulsive action between the pole and the 



