Wiedemann Effect in Ferromagnetic Substances. 655 



screw nut at one end of the bar in the axial line of the mag- 

 netizing coil, which was 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, 

 and a telescope. Since the Wiedemann effect is an odd 

 function of longitudinal or circular field, it is easily distin- 

 oiiishable from other effects such as the change of the modulus 

 of elasticity, which is an even function of the field. Prelimi- 

 nary experiments showed that the circular field has no effect 

 upon the modulus of elasticity, perhaps because the field is 

 not strong enough to cause such changes. Thev also showed 

 that the friction at the pivot is not sensible ; for the amount 

 of twist when the pivot was left free or when it was supported 

 gave almost coincident values. The direction of currents was 

 also so chosen that the rotation of the mirror causes contrac- 

 tion of the weak spring stretching the thin copper wire. 

 With the arrangement a twist amounting only to 1"'83 X 10~ 3 

 per cm. of our specimen was easily read. 



The measurement was conducted in the same order as in 

 the case of nickel steels. Here we noticed that a slight 

 residual magnetism considerably affected our results ; hence, 

 before each deflexion was taken demagnetization was carefully 

 effected . 



Twist by varying the longitudinal field. — Fig. 9 represents 

 the curves of twist per cm. in an iron bar plotted against the 

 external longitudinal field. Here c is the longitudinal our* 

 rent per square centimetre. The general course of the curves 

 is similar to that observed in the wire of the same metal. 

 Under a constant circular field, the angle of twist increases 

 at first slowly and then rapidly till it reaches a maximum in 

 a field of about 100 c.G.s. units ; it then diminishes and 

 ultimately changes its direction. The field in which the 

 twist reaches a maximum, and also the field of reversal, are 

 markedly larger in the bar than in the wire. Moreover, the 

 diminution of twist in the bar, after reaching a maximum, is 

 comparatively slow. These discrepancies will evidently dis- 

 appear if we take into account the demagnetizing force 

 acting in a direction opposite to that of the magnetizing 

 force. 



The results for nickel are drawn in ho-. 10 : the general 

 features of the curves are similar to those in the win 4 of the 

 same metal. The direction of twist is opposite to that of 

 iron in weak fields; but in strong fields the direction does not 



