ON THE MAGNETIC QUALITIES OF NICKEL. 



HABD-DRAWN Nickel Wire. 



After this the magnetising force was gradually removed and reversed, then re- 

 applied, then removed to zero, and then re-applied in the same direction as at first. 

 The curves in the figure show these subsequent actions sufficiently well to make it 

 unnecessary to quote the observed values of 3 and .$. After magnetising with 

 a force of 104 and getting an induced magnetism of 420, there was a residual 

 magnetism of 299, or 71 per cent, of the induced magnetism. It took a reversed 

 force of 18'5 to remove this. This is the quantity which HOPKINSON has called the 

 " coercive force." * The greatest susceptibility (K) was reached when $ was 24 and 

 3 was 270 ; its value is 11 '2, which gives 142 for the maximum permeability /A. The 

 energy dissipated through hysteresis in the large cycle, by double reversal of a 

 magnetising force of say 100 c.g.s. units ( J 3 d $ ), was 25,400 ergs. In simple 

 removal and re-application of the force without change of sign there was but little 

 hysteresis, and the dissipation of energy in that process was relatively insignificant. 

 The greatest value of the magnetic induction SB ( = 4w3 + < &) reached in this 

 experiment was 5380. 



The full lines in fig. 2 show in the same way the cyclic magnetisation of the same 

 piece of nickel wire after it had been annealed by heating to bright redness in the 

 flame of a Bunsen burner and cooling slowly in air. The greatest magnetisation now 

 reached under a force, ., of 100 is barely so high as before, but the susceptibility at 

 earlier stages is much greater, and the coercive force and the dissipation of energy are 

 much less. The following values refer to the first part of the process ; here, as in the 

 former case, there was some initial magnetism which the process of demagnetising by 

 reversals (applied before these observations were taken) did not remove : 



* HOPKINSON, loc. tit., p. 460. 



