1891.] 



on Electro-magnetic Repulsion. 



299 



When tlie magnet is demagnetised the copper experiences a momentary- 

 attraction. These attractions and repulsions are obviously due to 

 the Amperian stress set up between the magnet and the metal by- 

 reason of the induced currents set up in the latter. Impulsive 

 effects of this nature have been particularly studied by Mr. Boys.* 



§ 4. Let me, in the next place, ask you to contemplate a circuit, 

 say a closed conducting ring, suspended in front of the pole of an 

 electro-magnet, and that the coils of the electro-magnet are traversed 

 by an alternating current of electricity (Fig. 2). The magnetic field of 

 the magnet is then an alternating field. We shall suppose it to vary 

 in strength, according to a simple periodic law. The closed circuit 

 is therefore subjected to an inductive action, and we know that the 

 induced electromotive force in that circuit is at any instant propor- 



FiG. 2. 



Copper ring hung in the field of an electro-magnet, and repelled or attracted 

 when the current is put on or cut off. 



tional to the rate of change of the magnetic field in which it is 

 immersed. If, therefore, the variation in strength of that field is 

 represented geometrically by the ordinates of a periodic curve, the 

 varying electromotive force acting in the ring circuit is represented 

 by the ordinates of another such curve of equal wave length, shifted 

 a quarter of a wave length behind the first. In the diagram before 

 you the variation of the induced magnetic field, and the induced 

 electromotive force in the circuit, are represented, as usual, by two 

 harmonic curves. This induced electromotive force creates an induced 

 current flowing backwards and forwards in the ring, and we shall, in 

 the first instance, suppose that this current flows in exact synchronism 

 with its electromotive force. The induced current and the inducing 

 magnetic field may therefore be represented as to relative phase and 

 strength by the curves in the diagram (Fig. 3). The dynamical action, 



* See Proe, Phys. Soc. of London, vol. vi- p. 218, "A Magneto-electric 

 Phenomenon." 



