MAGNETIC PERMEABILITY OF IRON, STEEL AND NICKEL 53 



For comparison, I have added a column giving the values of Q which 

 would give the sustaining-power observed; some of these are as high 

 as any I have actually obtained, thus giving an experimental proof that 

 my estimate of 354 Ibs. cannot be far from correct, and illustrating 

 the beauty of the absolute system of electrical measurement by which, 

 from the simple deflection of a galvanometer-needle, we are able to 

 predict how much an electromagnet will sustain without actually trying 

 the experiment. 



TABLE IX. 



In looking over the columns of Table VIII, which contain the values 

 of the constants in the formula, we see how futile it is to attempt to 

 give any fixed value to the permeability of iron or nickel; and we also 

 see of how little value experiments on any one kind of iron are. Iron 

 differs as much in magnetic permeability as copper does in electric 

 conductivity. 



It is seen that in the three cases when iron bars have been used, the 

 value of a is negative; we might consider this to be a general law, if I 

 did not possess a ring which also gives this negative. All these bars 

 had a length of at least 120 times their diameter. 



The mathematical theory of magnetism has always been considered 

 one of the most difficult of subjects, even when, as heretofore, fj. is 

 considered to be a constant; but now, when it must be taken as a func- 

 tion of the magnetism, the difficulty is increased many fold. There are 

 certain cases, however, where the magnetism of the body is uniform, 

 which will not be affected. 



Troy, June 2, 1873. 



