MAGNETIC PERMEABILITY OF IROX, STEEL AND XICKEL 37 



of Green for the distribution of magnetism on a bar-magnet combined 

 with the known variation of K, that this can only be true for short and 

 thick bars; and it has also been remarked by Thomson that this should 

 be the case. 4 An experiment made in 1870 places this beyond doubt. 

 A small iron wire (No. 16), 8 inches long, was wound with two layers of 

 fine insulated wire; a small hard steel magnet inch long suspended by 

 a fibre of silk was rendered entirely astatic by a large magnet placed 

 about 2 feet distant; the wire electromagnet was then placed near it, 

 so that the needle hung H inch from it and about 2 inches back from 

 the end. On now exciting the magnet with a weak current, the needle 

 took up a certain definite position, indicating the direction of the line 

 of force at that point. When the current was very much increased, the 

 needle instantly moved into a position more nearly parallel to the 

 magnet, thus showing that the magnetism was now distributed more 

 nearly at the ends than before. This shows that nearly all the experi- 

 ments hitherto made on bar-magnets contain an error; but, owing to 

 its small amount, we can accept the results as approximately true. 



I believe mine are the first experiments hitherto made on-this subject 

 in which the results are expressed and the reasoning carried out in the 

 language of Faraday's theory of lines of magnetic force ; and the utility 

 of this method of thinking is shown in the method of experimenting 

 adopted for measuring magnetism in absolute measure, for which I 

 claim that it is the simplest and most accurate of any yet devised. 

 Whether Faraday's theory is correct or not, it is well known that its 

 use will give correct results; at the present time the tendency of the 

 most advanced thought is toward the theory 5 ; and indeed it has been 

 pointed out by Sir William Thomson that it follows, from dynamical 

 reasoning upon the magnetic rotation of the plane of polarization of 

 light, that the medium in which this takes place must itself be in 

 rotation, the axis of rotation being in the direction of the lines of 

 force. 8 Some substances must of necessity be more capable of assum- 

 ing this rotary motion than others; and hence arises the notion of 

 magnetic " conductivity '"' and " permeability." 



Thomson has pointed out several analogies which may be used in 

 calculating the distribution and direction of the lines of force under 

 various circumstances. He has shown that the mathematical treatment 



4 Papers on Electricity and Magnetism, p. 512. 



5 "On Action at a Distance," Maxwell, 'Nature,' Feb. 27 and March 6 and 13, 1873. 

 "Thomson's 'Papers on Electricity and Magnetism,' p. 419, note; and Maxwell's 



'Treatise on Electricity and Magnetism,' vol. ii, chap. xxi. 



