CHAXGE OF ELASTIC CONSTANTS OF FERROMAGNETICS. / 



by the relative expansion of the specimen and the compensating 

 wire. But to avoid the elVect of heating as much as possible, the 

 coil was waterjacketed. 



To increase the sensitiveness of the arrangement, a long 

 scale distance of 7.2*) m. was used. The scale w^as graduated on 

 a ground glass, and illuminated from behind by a mantled gas 

 flame. The telescope was placed in front of the apparatus and 

 near the rotating mirror ; consequently the line joining the cen- 

 ters of the scale and the mirror deviated slightly from the line 

 of sight of the telescope. In this way, the scale at such a long 

 distance could be read to a tenth of a millimeter with the observ- 

 ing telescope. 



If the specimen undergoes a differential e-ongation o/, a 

 rotation ocp of the axis is produced, such that d(p = <Vrlr, where r 

 is the radius of the axis. If the scale distance be d and the 

 reading on the scale corresponding to the elongation ul be '^ we 

 have 



n(p = -z-f ; hence ol=—j- 



In our case, 2?- ^1. HI mm., so that a deflection amounting to 

 one division of the scale in the field of the telescope corresponds 

 to an elongation of 4.47x10"' per unit length of the specimen. 

 The sensitiveness of the apparatus w\as therefore about 10 times 

 greater than that of the apparatus used by many previous investi- 

 gators. 



The magnetizing current supplied by a set of secondary bat- 

 teries w^as measured by a Siemens-Halske ammeter, which was 

 occasionally compared with a Kelvin ampere-balance. 



The present arrangement also enabled us to measure the 

 magnetic change of length of the specimen under constant tension 



