MAGNETISM AND TWIST IN IRON AND NICKEL. 383 



a hint as to the manner in which the current along the wire influences the phenomenon. 

 It would appear, in short, that the stronger the current along the wire the lower is the 

 tension which produces the maximum twist in a given longitudinal field. Hence, for 

 currents of considerable strength along the wire, it is quite possible that this maximum 

 twist, occurring at a tension lower than the lowest used, could not be observed. For it 

 must be remembered that in these experiments with a hanging wire, it is impossible to 

 begin at zero tension, since the dipping arrangement needed for making contact and other 

 necessary additions must have a definite weight. 



Thus we see that when nickel wire twists under the influence of circular and longi- 

 tudinal magnetising forces, the amount of twist in certain specimens is influenced by 

 tension in a manner very similar to what occurs in the case of iron — namely, the twist 

 diminishes as the tension increases. In other specimens, however, a maximum twist is 

 obtained for a certain intermediate tension ; and the tension which in a given longi- 

 tudinal magnetic field corresponds to the maximum twist appears to be smaller for 

 higher values of the circularly magnetising current along the wire. 



I am not aware that any experiments have been made upon the effect of tension on 

 the contraction of nickel in longitudinal magnetic fields. Mr Bidwell does not seem to 

 have studied the phenomena in nickel with anything like the thoroughness with which 

 he has worked out the phenomena in iron. All I can find is the statement that " a 

 nickel wire stretched by a weight undergoes retraction when magnetised," but whether 

 this retraction is greater or less than the retraction in the unstretched case is not 

 mentioned.* 



The tensions to which these various iron and nickel wires were subjected were 

 obviously carried beyond the approximate limits of so-called perfect elasticity. A very 

 natural inquiry to make was as to the effect of permanent strain upon the amount of 

 twist for a given combination of magnetising forces. In the experiments on the iron 

 wire of radius *032 (No. 2 in Table I.), observations were made as the tension was 

 diminished again to its first and lowest value. The numbers will be found in the table, 

 being in each case the last two rows of figures in the column. When the tension is 

 reduced to its original value, there seems to be a slight decrease in the twist ; but this 

 does not seem to hold for the intermediate conditions. All that can be safely said is 

 that the effect of the permanent strain, after the stress is removed, is hardly appreciable 

 in the case of iron. 



In the case of nickel, however, the effect of permanent strain is very marked, as the 

 following small table will show. In this table two pairs of columns of tensions and 

 twists are given. In the first of these are entered the tensions and twists before the 

 wire had been subjected to a high tension of above 6000 kgs. weight per sq. cm.; 

 and in the second column are entered the tensions and twists after the wire had been 

 subjected to this high tension and relieved. 



* See Proc. Roy. Soc, vol. xl. p. 133. 



