188 MAGNETISM 



There are two cases to be distinguished, that when the iron is 

 in a very weak field comparable with that of the earth, and that 

 when the field is very much stronger. 



With the weak field it will be observed that the permeability 

 increases enormously up to nearly 735 C. and then suddenly drops 

 apparently to that of air. With a field about ten times as great it 

 is at first more considerable, remains nearly constant for a time, 

 but drops gradually after 600 to the air value at 735 C. With 

 a field one hundred times as great it begins at the same value as 



IOOOO 



O 2OO AOO COO 



Temperature 



1 -'I... 135.* 



with the weak force, remains prurtirally constant to 500,and then 

 falls off to the condition of, apparently, no susceptibility at 735 C. 



Subsequent researches, which will be referred to in Chapter XXII, 

 have shown that the permeability does not fall quite to that of air. 

 Change of length on magnetisation. Joule f was tin- 

 first to observe that a bar of iron changes its length on magnet Na- 

 tion. He placed the bar or rod to be experimented on within a 

 magnetising coil with its lower end fixed and its upper end prrxxincr 

 against a lever. This in turn pressed against a second lever, of 

 which the free end moved in the field of a microscope. One 

 division of the microscope scale corresponded to about 1/140000 

 of an inch. The rod was 36 inches long, and he observed lengthen- 

 ing, increasing with the magnetisation, up to twenty-eight divisions, 

 or 1/180000 of the length. He used only comparatively small 

 magnetising forces, and as far as he went the lengthening was as 

 the square of the magnetising force. Joule further sought to 

 measure any change in the total volume of the iron by immersing 

 the rod in a tube filled with water and provided with a stopper 



* Hopkinson, Phil. Trans. A. (1889), Plate 16. 

 f Scientific Papers, i. p. 235. 



