154 Hoy at Society: — 



the strength of the magnetizing current was increased. On di- 

 viding the increase of resistance by the square of the magnetizing 

 current, the numbers obtained from a considerable number of ex- 

 periments lie between 4*7 and 5*6, showing that the ratio of the 

 increase of resistance to the square of the magnetizing current 

 is very nearly constant. 



When the magnetizing current is considerably increased, this 

 ratio is foimd to diminish, just as in the case of soft iron. 



Different kinds of hard steel were tried. 



(1) An ordinary knitting-needle, of which the resistance was 

 66*5 units. 



On magnetizing with currents of different strengths, there was 

 found to be a diminution in the resistance ; and it was also found 

 that the diminution of resistance increased when the strength of the 

 current was increased. With currents varying from tan 15° to 

 tan 54° 30' the diminution amounted to 4-33, i. e. about 6*5 per 

 cent, of the whole resistance. The temperature increased about 2 C 

 C. during the experiment. 



Dividing the loss of resistance by the square of the mag- 

 netizing current, the results of four sets of experiments gave 

 the following values : — 



(tanlS ) 2 -^ 

 52 |_=2-26, 



(tan 30°)* 

 2-3225 



— =2*24 

 (tan45°30') 2 



4-330 =2 . 21 



(tan54°30') 2 



Four Grove's cells were employed for the strongest current. 



Two other experiments which had been tried previously gav< 

 results 2*30 and 2-26 for the ratio of the diminution of resistance 

 to the square of the magnetizing current, thus showing that the 

 diminution in the resistance is almost exactly proportional to the 

 square of the current. 



The diminution in resistance does not take place all at once 

 but gradually, and also ceases gradually when the current is 

 stopped. 



(2) A steel needle was also magnetized longitudinally by placing 

 it on a copper strip at right angles to the lines of force of a current 

 across the strip. 



There was found to be diminution of resistance on increasing 

 the current. The values obtained from two series of experiments 

 were : — 



•° 64 ^2-88, 



(tan 8° 30') 



•192 



(tan 15°) 



=2-63. 



