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



experiment, but illustrates what extremely small changes will 

 affect the permeability of any metal. 



We have now completed the discussion of the results as far 

 as they refer to the magnetic permeability, leaving the discus- 

 sion of the temporary and permanent or residual magnetism to 

 the future, although these latter, when discussed, will throw 

 great light upon the nature of the coercive force in steel and 

 other metals. The whole subject seems to be a most fruitful 

 one, and I can hardly understand why it has been so much neg- 

 lected. It may have been that a simple method of experiment 

 was not known ; but if so, I believe that my ^method will be 

 found both accurate and simple, though it may be modified to 

 suit the circumstances. Professor Maxwell has suggested to me 

 that it would be better to use rods of great length than rings, 

 because that in a ring we can never determine its actual mag- 

 netization, but must always content ourselves with measuring 

 the change on reversing or breaking the current. This is an 

 important remark, because it has been found by MM. Marianini 

 and Jamin, and was noticed independently by myself in some 

 unpublished experiments of 1870, that a bar of steel which has 

 lain for some time magnetized in one direction will afterwards 

 be more easily magnetized in that direction than in the other. 

 This fact could not have been discovered from a ring; and in- 

 deed if a ring got a one-sided magnetism in any way we might 

 never know it, and yet it might affect our results, as indeed we 

 have already seen in the case of the magnetic curve. But at the 

 same time I think that greater errors would result from using 

 long bars. I have tried one of iron 3 feet long and \ inch dia- 

 meter ; and the effect of the length was still apparent, although 

 the ratio of length to diameter was 144. To get exact results it 

 would probably have to be several times this for the given spe- 

 cimen of iron, and would of course have to be greater for a piece 

 of iron having greater permeability. This rod must be turned 

 and must be homogeneous throughout — conditions which it 

 would be very difficult to fulfil, and which would be impossible 

 in the case of nickel and cobalt. We might indeed use ellipsoids 

 of very elongated form ; and this would probably be the best of 

 all, as the mathematical theory of this case is complete, and it is 

 one of the few where the magnetization is uniform, and which 

 consequently will still hold, although the permeability may vary 

 with the amount of magnetization. This form will, of course, 

 satisfy Professor Maxwell's objection. 



The method of the ring introduces a small error which has 

 never yet been considered, and which will affect Dr. Stoletow's 

 results as well as mine. The number of lines of induction pass- 

 ing across the circular section of a ring-magnet we have seen 



