MAGNETIC PERMEABILITY OF IRON, STEEL AND NICKEL 49 



part of the curve by the broken line. 20 I believe that I have obtained 

 at the least a very close approximation to the true equation of the curve, 

 and will show further on that Q and M must satisfy the equation 



D 



It is very probable that Weber's theory may be so modified as to 

 give a similar equation. 



Space will not permit me to discuss the curves of temporary and 

 permanent magnetism; but I will call attention to the following facts 

 which the Tables seem to establish. 



1. Nearly or quite all the magnetism of a bar is, with weak magnetizing- 

 forces, temporary; and this is more apparent in steel than in soft iron. 



2. The temporary magnetism increases continually with the current. 



3. The permanent magnetism at first increases very fast with the current, 

 but afterwards diminishes as the current increases, when the iron is near 

 its maximum of magnetism. 21 



I have now described the methods of plotting the Tables hitherto 

 used; and I will now describe the third, which is, I believe, new. This 

 is by using the values of the magnetism of the bar as abscissas, and 

 those of the permeability as ordinates. In this way we obtain a per- 

 fectly regular curve, which is of finite dimensions, and from which the 

 maximum of magnetism can be readily obtained. Plate III shows this 

 method of plotting as applied to Table I. If we draw straight lines 

 across the curve parallel to the axis of Q and mark their centres, we 

 find that they always fall very exactly upon a straight line, which is 

 therefore a diameter of the curve. The curve of nickel shown upon 

 the same Plate has this property in common with iron. I have made 

 several attempts to get a ring of cobalt; but the button has always 

 been too porous to use. However, I hope soon to obtain one, and thus 

 make the law general for all the magnetic metals. There are two 

 equations which may be used to express the curve : one is the equation 

 of an inclined parabola; but this fails for the two ends of the curve; 

 the other is an equation of the general form 



(11) 



20 Treatise on Electricity and Magnetism, Maxwell, vol. ii, chap. vi. 



21 The last clause of this sentence cannot be considered yet as entirely settled, 

 though I have other curves than those shown here which show it well. [This note 

 was added to the original paper by Professor Rowland.] 



4 



