and Maximum of Magnetism of Nickel and Cobalt. 335 



Hence the place of greatest permeability will vary with the kind 

 of metal. From these, however, we can approximate to the 

 value of h in the formula; for we have 



, 27,000 ' 11,000 



for iron, o = — -= — ; for Nickel, b = — ^ — ; 



for Cobalt, 6 = 26,000. 



In Table V. we have the results for cobalt in the magnetic 

 state. We here find the same effect of magnetization as we have 

 before found for iron and nickel. 



In Table VI. we have results for cobalt at a high temperature, 

 and see how greatly the permeability is increased by rise of tem- 

 perature, this being for the vertex of the curve about 70 per 

 cent. But on plotting the curve I was much surprised to find 

 an entire departure from that regularity which I had before 

 found in all curves taken from iron and nickel when the metal 

 was homogeneous. At present I am not able to account for this, 

 and especially for the fact that one of the measurements of 23 Js 

 higher than that which we have taken for the maximum of mag- 

 netization, at, however, a lower temperature. The curve is 

 exactly of the same nature as that which I have before found 

 for a piece of nickel which had been rendered unhomogeneous by 

 heating red-hot, and thus burning the outside. The smaller 

 cobalt ring gives a curve of the same general shape as this, but 

 has the top more rounded. I will not attempt without fresh 

 experiments to explain these facts, but will simply offer the* fol- 

 lowing explanations, some one of which may be true. First, it may 

 be due to want of homogeneity in the ring; but it seems as if 

 this should have affected the curve of Table IV. more. Secondly, 

 it may be at least partly due to the rise in temperature of the 

 ring at high magnetizing-powers ; and indeed we know that this 

 must be greater in paraffin than in alcohol for several reasons : 

 there is about twice as much heat generated in copper wire at 

 230° C. as at 0° with the same current; and this heat will not 

 be conducted off so fast in paraffin as in alcohol, on account of 

 its circulating with less freedom ; it probably has less specific 

 heat also. Thirdly, it may be due to some property of cobalt, 

 by which its permeability and maximum of magnetization are 

 increased by heat and the curve changed. 



The experiments made with the small ring confirm those 

 made with the large one as far as they go ; but as it was so 

 small, they do not possess the weight due to those with the 

 larger one. But, curious as it may seem, although they were 

 turned from the same button side by side, yet the permeability 

 of the larger is about 45 per cent, greater than that of the 

 smaller. I have satisfied myself that this is clue to no error in 



