CHANGE OF Er.ASTIC CONSTANTS OF FERROMAGNETICS. 45 



reilectiiig iiiirror and bent downward, rlust below it, a small 

 mercury cup was placed, into which the wire dipped. 



Our method of observation was as follows : — First the torsion 

 circles were adjusted so that the magnetic field produced no de- 

 flection of the mirror. The readings of both circles for this posi- 

 tion corresponded lo those of the untwisted state of the wire. 

 Next the lower circle was twisted by 6 and then the upper circle 

 was tv>-isted in the opposite sense, until the mirror regained its 

 original position for no field. Let the angle through which the 

 upper circle was twisted be denoted by d'; 6' was nearly equal to 

 d. After demagnetization by reversals, the mirror usually turned 

 through a small angle ; but the amount of the rotation was 

 usually so small that we could leave it out of account. Different 

 fields were successively applied and corresponding deflections were 

 read off". Next the twist was increased and the procedure repeated. 



The experiments were made also for different tensions. Be- 

 fore increasing the tension, the wire Avas untwisted to the original 

 state of no twist, after passing through a cyclic twist of gradual- 

 ly decreasing amplitude about the position of no twist. This 

 procedure was always necessary to restore the wire to the state of 

 no twist, since the simple untwisting usually left a residual deflec- 

 tion. 



If the couples required to give unit twist to the two wires 

 be r and r' respectively, we have 



if the ansular deflection of the mirror due to magnetization be 

 ddf and ^r the increment of r, we have 



• • 7ji = z + oz and {z^rn){d-dd)=T'{d'-^od) 



or Zj^{d-<W) = z\0'-\-oO) und r,T{i}-üt)) = oO{z' +t), 



