Wiedemann Effect m Ferromagnetic Substances. 653 



oscillation was rendered very small, especially in the case 

 when a weight was attached. To stop the oscillation a brass 

 wire was fixed horizontally to the vertical wire and bent 

 downward, as shown in the annexed figure. Just below it a 

 small mercury cup was placed ; this cup was connected with 

 a large one by a caoutchouc tube. This large cup was placed 

 near the observers and could be raised or lowered by means 

 o£ a screw adjustment. The motion caused the mercury in 

 the small cup to be raised or lowered, so that the side wire 

 dipped into the mercury or hung free. When we wished to 

 stop the oscillation of the wire, the side wire was dipped into 

 the mercury in the small cup, while the reading was always 

 taken with the wire hanging free of the mercury. 



The experiment was conducted in the following manner: — 



1. The circularly magnetizing current was kept constant, 

 and the amount of twist due to varying the longitudinally 

 magnetizing current was measured. 



2. The wire was then stretched by different loads and the 

 above processes were repeated. 



3. The longitudinally magnetizing current was kept con- 

 stant, and the amount of twist due to varying the circularly 

 magnetizing current was measured. 



Before each experiment care was taken to demagnetize the 

 wire completely either longitudinally or circularly by passing 

 an alternate current of gradually diminishing intensity. This 

 was found absolutely necessary to secure correct results. 



Twist by varying the longitudinal field. — If the direction of 

 the longitudinal field is right-handedly related to that of the 

 circular field, nickel steel is twisted in the direction of the 

 latter. As shown in PL XVI. figs. 1 & 2, under a given circular 

 field, the amount of twist at first increases till it reaches a 

 maximum, after which it gradually diminishes. But the re- 

 versal of the twist is never observed, though the field exceeds 

 1200 C.G.s. units. The position of the maximum twist is 

 slightly displaced in high fields as the longitudinal current 

 increases. The amount of twist is greater in 45 per cent. 

 nickel steel than in 85 per cent, nickel steel. 



In the experiment above cited, Prof. Nagaoka and one of 

 us observed in some cases the reversal of the direction of 

 twist in 45 per cent, nickel steel ; but in the present experi- 

 ment we did not notice this reversal of twist. 



The effect of tension. — The effect of tension on twist in 

 nickel steels is not so marked as that of tension on the mag- 

 netic change of length in the same metal. As seen from 

 figs, o & 4, the tension always diminishes the amount of 

 twist ; the diminution is large in weak fields, and becomes 



