Wiedemann Effect in Ferromacpietic Substances. 651 



magnetic bars and the effect o£ torque on it. We lately 

 published a paper* relating to the effect of tension on the 

 magnetic change of length ; in this we found that the mag- 

 netic elongation of nickel steels is largely affected by tension, 

 and that when the tension exceeds a certain value, the con- 

 traction is accompanied by magnetization. From Maxwell's 

 explanation as well as that of Kirchhoff for the Wiedemann 

 effect, it seems probable that the direction of twist in nickel 

 steels is reversed when the suspended weight exceeds the 

 said limit. We therefore studied this point particularly and 

 found that the above inference is not correct. So far as we 

 were aware the Wiedemann effect in cobalt had not yet been 

 studied, perhaps because it is difficult to obtain a specimen 

 in the form of a wire on account of its brittleness. It was 

 therefore desirable to have an experiment for the metal. 

 Our apparatus f used in studying the change of rigidity by 

 magnetization was conveniently used for examining the 

 Wiedemann effect in ferromagnetic bars. We had two co- 

 balt bars, one in the cast state and the other in the annealed. 

 The observations showed that the twist in cobalt was just the 

 reverse of that of iron, as was to be expected from its change 

 of length by magnetization. 



We tested eight different examples shown in the following 

 table :— 



Metal. 



Lengtb. 



Diameter. 



cm. 



00956 



: 45 per cent, nickel steel ... 



cms. 

 20-80 



)> >> »> ••• 



20-90 



00516 



35 per cent, nickel steel ... 



20-92 



00939 



„ ••• 



2096 



00509 



Soft iron bar 



21-03 

 2100 

 21-00 



1004 

 1-117 

 1-038 



Nickel bar 



Oast cobalt bar 



Ann. cobalt bar 



2100 



1082 



• 



Our arrangement for studying the Wiedemann effect in 

 nickel-steel wires was the same as that used by Prof. Nagaoka 

 and one of us in a former experiment. 



To the extremities of a nickel-steel wire 21 cms. long were 

 brazed stout brass wires, and a light mirror was attached to 



* Phil. Mag. iv. p. 338 (1902). 

 f Ibid. iv. p. 537 (1902). 



