ON THE MAGNETIC QUALITIES OP NICKEL. 



329 



zero, and noting the residual value of 3 (which will be distinguished as 3r), then 

 applying a stronger magnetising force, and so on. By this means curves were drawn 

 showing the relation of the induced magnetism 3 to the force , and also the residual 

 magnetism 3r to the force $, in each of the three conditions of stress. Before passing 

 from one to another load, the wire was demagnetised by reversals, and this process 

 was repeated after the new load had been put on. This precaution was taken because it 

 had been found in experiments on iron that the exact form which the curve of 3 and . 

 took under any assigned load depended on whether the process of demagnetising 

 had or had not been performed after the load was applied.* 



The results of these experiments with nickel wire in the hard-drawn state are 

 given below, and are shown in fig. 3. In the Table a column is added to show the 

 ratio of residual to induced magnetism. 



HARD-DRAWN Nickel Wire. 



As the section of the wire was 0'363 sq. mm., each kilogramme of load corresponds 

 to a stress of 2'75 kilogrammes per sq. mm. 



These results agree with Sir WILLIAM THOMSON'S experiments, which were made by 

 loading and unloading a nickel rod exposed to constant magnetising forces, in showing 

 that longitudinal pull reduces the magnetic susceptibility of nickel. The reduction is, 

 in fact, enormous even under so moderate a load as 1 2 kilos, (or 33 kilos, per sq. mm.), 

 a load well within the elastic limit of the wire.t Great as the effects of stress are on 

 the induced values of 3 they are still greater on the residual values, so much so that 

 a load of 12 kilos, may be said almost to do away with the retentiveness of the wire 

 with respect to such magnetising forces as the experiments deal with. In magnetisa- 

 tion by very low forces nickel, like iron, retains sensibly none of the induced magnetism 

 when the force is withdrawn, and one effect of longitudinal pull is to extend the range 

 of magnetising force for which this is true. 



Ewnra, loc. cit., 96-105. . 



t A specimen of this wire, which was loaded until it broke, showed little elongation until a load of 

 18 kilos, was reached. It broke with 23 kilos., after extending 9 per cent. 

 MDCCCLXXXVIII. A. 2 U 



