DESTRUCTION OF SUBPERMANENT MAGNETISM. 171 



77. Reversion or destruction of the magnetism. 

 Origin of the term l Subpermanent.' 



Suppose that the lower end of the bar in the ex- 

 periment of Article 75 is distinguished by being 

 painted white. This white end, after the bar has 

 been placed with its white end in the direction of 

 dip and has been struck, is found to be charged with 

 red magnetism. Now reverse the bar upon the dip 

 plane with its white end upwards, and strike it; it 

 will be found that its white end is charged with 

 blue magnetism. The magnetism has been reversed. 

 Undoubtedly, in order to arrive at this state, it has 

 gone through the stage of being destroyed or rendered 

 undiscoverable by instruments. 



But the pure destruction may be visibly effected 

 in the following manner. Lay the bar upon that 

 surface of the magnetic anvil which is normal to the 

 direction of dip, and strike it with a few blows of the 

 hammer. On removing the bar, and testing its state 

 by means of a compass, it will be found that all trace 

 of magnetism has disappeared. The bar is now in the 

 same state as before the experiment of Article 75. 



If, however, the magnetized bar be subject to no 

 such violence, but be suffered to rest quietly, or be 

 moved gently into different positions, it will slowly 

 lose a large proportion of its magnetism. And it is 

 this peculiar character which necessitated the intro- 

 duction of a new name. The magnetism of a struck 

 iron bar resembles the magnetism of a permanent 



