MAGNETISM AND TWIST IN IRON AND NICKEL. 381 



2. The longitudinal field necessary to produce the maximum twist is greater for the 

 greater current along the wire. This result also may be shown to be in harmony with 

 Maxwell's explanation. 



3. For a given combination of magnetising forces, the twist diminishes steadily as the 

 tension increases. This relation also holds for all the other combinations of magnetising 

 forces used in the experiments. The conclusion is in remarkable agreement with the 

 results obtained by Mr Bid well in his elaborate investigations into the changes of length 

 of iron in magnetic fields. Amongst other results, he found that under increasing loads 

 the elongation of iron wire due to moderate magnetising forces decreases.* Here again 

 Maxwell's explanation of the Wiedemann effect, in terms of the simpler Joule effect, 

 accords well with the facts — both being powerfully influenced by tension, and that in the 

 same direction. 



At first sight, it might be supposed that the maximum twist shown to exist in these 

 experiments was exactly the same phenomenon as the maximum elongation of iron, 

 obtained by Mr Bidwell in his experiments. But in trying to connect the two pecu- 

 liarities, we encounter discrepancies which are hard to explain. Thus, the field which 

 corresponds to the maximum elongation is much higher than the field which corresponds 

 to the maximum twist. The latter, of course, depends on the strength of the current 

 along the wire ; but in no case (out of nearly 200 distinct experiments with different 

 wires) has the magnetic field corresponding to the maximum twist been higher than 25 

 electromagnetic units — usually considerably lower, as in the table given above. Accord- 

 ing to Mr Bid well's experiments, the field producing maximum elongation in a wire 1 '2 

 mm. in diameter, varied from 45 for low loads to about 20 for high loads. This very 

 striking effect of increased tension upon the strength of field required to produce the 

 maximum elongation has, further, no analogue in the experiments on the maximum 

 twist. Again, as bearing on this point, it may be mentioned that, although it is possible 

 to obtain a maximum twist in nickel, there is no evidence of a maximum contraction. 

 Hence the existence of the maximum twist does not imply the existence of maximum 

 elongation or contraction, but clearly depends on other considerations. As I hope to 

 show later, however, these considerations seem to be necessarily involved in the complete 

 statement of the Maxwell explanation. 



We shall now pass to the discussion of the results for the nickel wires. Here we are 

 not able to make use of such a well-marked singular point as a maximum, since no 

 maximum was obtained with the magnetic fields employed. The main purpose of the 

 present inquiry is, however, to consider the effect of tension on twist, other things being 

 the same. It suffices, therefore, to fix upon some one value of the longitudinal field, which 

 is common to all the experiments. In the following tables, only those twists are given 

 which correspond to the field 28 "5. In the great majority of cases this was in reality 

 one of the fields employed ; and in the comparatively few instances where it was not so, 

 it was an easy matter to obtain by a simple interpolation an accurate enough value for 



* See Proc. Roy. Soc, vol. xl. p. 262 (1886). 



