EFFECT OF STRESS ON MACiXETIZATlUX. 5 



zation. To obtain a smooth motion of tlic i;;ilvjinometer niiiror 

 due to the induction, it was found necessary thnt the two primaries 

 as well as the two secondaries should have nearly the same 

 dimensions respectively ; the kick, which was observed when the 

 dimensions of these coils were di fièrent, was probably due to the 

 self-inductions and the capacities in these coils. 



Compensation for the earth field was eöected by a special 

 coil of fine copper wire wound on a glass tube in a single layer. 

 This coil fitted closely to the inside of the magnetizing coil and 

 to the outside of the secondary. It was fed by a current from 

 2 Daniel cells with adjustable resistance in the circuit. 



The current in the primary circuit was measured by a 

 ►Siemens and Halske ammeter witli two shunts, -w- and 7^77. 

 This was occasionally compared with a Kelvin's ampere-balance. 



The specimen to be tested was cut to a suitable length (about 

 27 cm.) so that if placed centrally, it might lie in a nearly uni- 

 form field of the magnetizing coil ; it was brazed at its both ends 

 to thick rods of brass. The whole was hung vertically in the 

 axial line of the magnetizing coil, and consequently of the second- 

 ary coil, the upper rod being firmly clamped to the rigid frame 

 above the coils. To the end of the lower rod, a hook was attached ; 

 from this hung a flexible cord which, after passing through a 

 system of two pulleys, was stretched by a weight, without im- 

 parting any injurious pendulum motion to the specimen. Near 

 the end of the lower rod, a rigid pin was screwed on perpendi- 

 cular to the rod. The ends of the pin fitted to the two V-shaped 

 grooves cut lengthwise and diametrically oj^posite to each other 

 on the inside of a brass cylinder, which could be turned about a 

 fixed vertical axis to any desired angle. The angle of twist was 

 read by means of a graduated circle and the index attached to 



