330 Mr. H. A. Rowland on the Magnetic Permeability 



perties of nickel are a complete reproduction of those of iron on 

 a smaller scale. But when we come to study the effect of tempe- 

 rature we shall find a remarkable difference, and shall find nickel 

 to be much more susceptible than iron to the influence of heat. 



In Table III. we have experiments on the permeability of 

 nickel at a high temperature, the ring being maintained at 

 220° C. by being placed in a bath of melted paraffin : in this 

 bath the silk covering of the wire remained quite perfect, but 

 after many hours became somewhat weak. After completing 

 the experiments on this and the cobalt rings, on unwinding 

 some of them I found the outside layer quite perfect ; but, espe- 

 cially in the smallest ring, the silk on the inside layer was much 

 weaker, although the insulation was still perfect when the wire 

 was in place. I can only account for this by the electric current 

 generating heat in the wire, which was unable to pass outward 

 because of the outside layer and also of the pieces of paper 

 which were used to separate the layers of wire ; hence the ring 

 at high magnetizing-powers must have been at a somewhat 

 higher temperature than the bath, to an amount which it is im- 

 possible to estimate. It is probable that it was not very great, 

 however ; for at this high temperature continued for hours it 

 requires but little increase of heat to finally destroy the silk. 

 We can, however, tell the direction of the error. 



We see, on comparing Tables I. and II. with Table III., the 

 great effect of heat on the magnetic properties of nickel. We 

 see that for low magnetization the permeability is greatly in- 

 creased, which is just opposite to what we might expect ; but on 

 plotting the curve we also notice the equally remarkable fact, 



SO.OCfi. 



1. Curve at 15° C. 



2. Curve at 220° C. 



that the maximum of magnetization is decreased 'from 53 = 63400 

 or 3 = 4940 to 35 = 49000 or 3 =3800. This curious result is 

 shown in the annexed figure, where we see that for low magne- 



