86 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



magnetizing field, it returns almost completely to its initial con- 

 dition when the weak field ceases to act. A bar of iron 

 ot steel, which is placed in a strong magnetizing field, re- 

 turns almost completely to its initial condition when the field 

 is slightly increased and then decreased again. That is, a bar 

 of iron or steel exhibits a kind of magnetic elasticity. This 

 action is especially prominent in hardened steel. Thus, a small 

 magnet ns, Fig. 49, is repelled by the strong north pole N 

 of another magnet. But when the small magnet is brought very 

 near to N, as shown in Fig. 50, its magnetism is reversed and 



Fig. 49. Fig. 50. 



it is attracted by N, and then, if the reversal of magnetization 

 of ns has not been carried too far, it will be found to be again 

 repelled by N when it is removed to the position shown in 

 Fig. 49. This shows that the magnetism of ns after having 

 been actually reversed by the field near N returns approxi- 

 mately to its initial value when this reversing field ceases to act. 

 This is analogous to the following : A flat steel spring is fixed in 

 a vise and bent sufficiently to give it a permanent set to the left, 

 a force is then exerted on the rod bending it to a slight extent to 

 the right and when this force ceases to act, the rod again takes 

 on its " permanent " bend to the left. 



Eiving's theory. The apparent frictional and elastic opposition 

 to the turning of molecular magnets may both be ascribed to the 

 mutual action of these molecules as magnets. This was first 

 pointed out by Ewing* who constructed a model consisting of a 

 large number of small magnets supported on jewels and pivots 

 and arranged on a board. When this system of magnets is sub- 

 jected to the action of a weak magnetic field, each magnet is 

 slightly turned, and every magnet returns to its initial position 



* See Philosophical Magazine, series 5, Vo). 30, page 205. 



