WIEDEMANN EFFECT IN FERROMAGNETIC SUBSI'ANCES. 7 



Here C denotes the current per squai'e niiJlimeter, H the external 

 field and ä the angle of twist per centimeter. In the experiment 

 above cited, Professor Nagaoka and one of us observed in some cases 

 the reversîd of the direction of twist in 45% nickel steel ; but in 

 the present experiment, we did not notice this reversal of twist. 



Tlie efect of tension. The etfect of tension on twist in nickel 

 steels is not so marked as that of tension on the maofnetic chancre of 

 length in the same metal. As seen from Figs. 3 and 4, the tension 

 alw;ivs diminislies the amount of twist: the diminution is larofe in 

 weak fields and l)ecomes gradually less as the field is increased, till it 

 becomes insensibly small. The diminution is approximately propor- 

 tional to the applied tension. 



To test the elfect of heavy loading, thin wires about |mm. thick 

 were examined. Even by a tension in which contraction occurs by 

 magnetization, the direction of torsion in nickel steels is not reversed, 

 though the amount of the maximum twist is reduced to about ^ or i 

 its value corresponding to no tension, as seen from Figs. 5 and 6. 



Whichever theory we adopt, whether Maxwell's or Kirchhofes, 

 the direction of twist is principally determined by the sign of the 

 quantity SÀ—a^ where ^^ and ö" are respectively the length- and the 

 volume -chanofe of the ferromaofnetics. When there is no tension 

 acting (311 the wire, the sign of SX— a must be positive, because the 

 direction of twist in the alloy is the same as that of iron. By applying 

 a heavy load, the contraction is accompanied by magnetization so that 

 X is negative. Hence in order that S?. — <7 should be positive, <t must 

 necessarily be negative under heavy loading ; that is, the ch;mge of 

 volume by magnetization must change its sign from positive to 

 negative, as the load is increased. 



