64 HENEY A. ROWLAND 



Suppose we have values of 33, and wish to find the corresponding values 

 of .//. We first calculate a few values of 33 from (2) so that we can plot 

 the curve connecting 33 and [JL. We then from the plot select a value 

 of p which we shall call //, as near the proper value as possible, and 

 calculate the corresponding value of 33, which we shall call 33'. Our 

 problem then is, knowing 33' and //, to find the value of /JL corresponding 

 to 33 when this is nearly equal to 33'. Let 33' receive a small increment 

 J33', so that 33 = 33' + J33' ; then we have, from Taylor's theorem, since 

 ' + J33') and fjf= 



Remembering that the constants in (1) refer to degrees of arc and 

 not to the absolute value of the arc, we have 



&c, 



which is in the most convenient form for calculation by means of 

 Barlow's Tables of squares, &c., and is very easy to apply, being far 

 easier than the method of successive approximation. 



On comparing the magnetic curve Table II with the normal curve 

 Table I, we see that the magnetic curve of nickel bears the same rela- 

 tion to the normal curve as we have already found for iron; that is, 

 the magnetic curve falls below the normal curve for all points before 

 the vertex, but afterwards the two coincide. 



Hence we see that at ordinary temperatures the magnetic properties 

 of nickel are a complete reproduction of those of iron on a smaller scale. 

 But when we come to study the effect of temperature 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 some- 

 what weak. After completing the experiments on this and the cobalt 

 rings, on unwinding some of them I found the outside layer quite per- 

 fect; but, especially 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 



