1898 - 99 .] Prof. Knott on Magnetic Twist in Nickel Tubes. 595 
smaller twist — in other words, the result is as if a less powerful H 
acted without hysteresis. 
Thus we see the reason why, when H is greater than H' (as 
is generally the case), the twist for steady H' and cyclically-varying 
±H is greater than for steady H and cyclically-varying ±H'. 
On the other hand, for small values of H the contrary should hold. 
This result could not be established in the present experiment with 
the nickel tube, because of the minuteness of the effect in small 
longitudinal fields; but it was obtained eight years ago in my 
experiments with wires (see Trans. Roy. Soc. Edin ., vol. xxxvi., 
1891). The curves published with that paper show the result 
very beautifully ; for in the lower fields the curve giving the 
twists in ascending longitudinal fields, when the circular field is 
varied cyclically, lies above the curve giving the twists in the 
same combinations of field when the longitudinal field is varied 
cyclically. At a certain field the curves cross, and the first-named 
curve continues ever after to be the lower. The critical field in 
which the twist is the same, however the combination of fields 
may be applied, is greater as the circular magnetizing field is 
increased — a conclusion quite in accord with the views now 
stated. 
The fact that the value of the twist, calculated by means of the 
formula given above, is intermediate to the two observed values 
obtained experimentally (1) with cyclical variation of the longi- 
tudinal field, (2) with cyclical variation of the circular field, is a 
result of some importance. It demonstrates the sufficiency of 
the explanation given by Maxwell that the Wiedemann effect is 
essentially the Joule effect. In other words, in striving to get an 
insight into the molecular changes that accompany magnetization, 
we are not warranted in considering the twist effect observed by 
Wiedemann as involving any factor that is not prominent in the 
production of the elongation effect observed by Joule. 
