of Steel, Nickel, Cobalt, and Nickel-Steels. 61- 



compensated by placing another coil in the interior of the 

 magnetizing coil. The lower part of the wire to be tested 

 was protected against air- currents by inclosing it in a wide 

 brass tube with a small window, just where, the reflecting 



Tier. 10. 



J 



J/ , 



T?F* 



mirror was attached. The twist was measured by scale and 

 telescope method, by which the deflexion of 0"'3 per cm. 

 was easily read. The current was measured by Kelvin graded 

 amperemeters, whose constants were from time to time 

 checked by means of an ampere balance. Before each 

 experiment, care was taken to demagnetize the wire completely, 

 either longitudinally or circularly, by passing an alternate 

 current of gradually diminishing intensity. 



Twist by varying the longitudinal field. — The direction of 

 twist in iron, so long as the longitudinal magnetizing field is 

 not strong, is such that if the current is passed down the wire 

 i'rom the fixed to the free end and the wire is magnetized 

 with north pole downwards, the free end, as seen from above, 

 twists in the direction of the hands of a watch. By keeping 

 the circular field constant, the amount of twist increases at 

 first, till it reaches a maximum in a field of about 20 units 

 (fig. 11) ; it then goes on diminishing till it ultimately changes 

 direction and continues to twist in the opposite direction with 

 increasing fields. The field at which the twist is reversed 

 increases with the circularly magnetizing field. In nickel, 

 the direction of twist is opposite to that in iron, but the 

 general feature is similar to iron, the only difference being 

 that even in strong longitudinal fields the twist is not reversed. 

 For wires of equal thickness, the amount o£ twist in nickel 

 is greater than in iron — the maximum twist in iron wire 



