WIEDEMANN EFFECT IN FERROMAGNETIC SUBSTANCES. 



?yh% nickel steel. 



The apparatus for studying the Wiedemann effect in ferromagnetic 

 bars was that used in the experiment on the change of rigidity by 

 magnetization ; the longitudinal current was led to the bar by means 

 of mercury contact without causing sensible resistance. The ferro- 

 magnetic bar was soldered on both ends to brass bars of a thicker 

 diameter, as in the former experiment just referred to. It was fixed, 

 by means of the screw nut at one end of the bar, in the axial line of 

 the magnetizing coil, which \vas placed magnetic east and west. The 

 pivot at the other end of the bar carrying a double wheel was lightly 

 placed in contact with the agate cup fixed to the wooden frame. The 

 twist was measured by means of a rotating cylinder with a reflecting 

 mirror, a vertical scale !ihd a telescope. Since the Wiedemann effect 

 is an odd function of longitudinal or circular fields, it is easily dis- 

 tinguishable from other effects such as the change of elasticity or that 

 of rigidity, which is an even furiction of the field. Preliminary ex- 

 periments showed that the circular field has no effect upon the modulus 

 of elasticity or of rigidity, perhaps because the field is not strong 

 enousrh to cause such chano-es. They also showed that the friction 

 at the pivot is not sensible ; for the amount of twist when the pivot is 

 left free or when it is supported, gave almost coincident values. The 



