MEASUREMENT OF PERMEABILITY 



275 



is continuous within and without the iron there, and Ewing 

 calculated that if the pole pieces had an angle of 78 28' the field 

 would be nearly uniform as well as continuous. In order to 

 measure the intensity H of the field a second coil of somewhat 

 larger area was wound round the first, the difference in areas being 



FIG. 211. 



known. The difference in the galvanometer throw when the 

 second coil was used and when the first coil was used gave the 

 value of H in the air, and therefore in the iron. 



The following Table (Ewing, loc. cit. p. 150) gives the results 

 obtained with a specimen of Swedish iron, and will serve as a 

 type : 



All the specimens examined showed the same tendency towards 

 a limiting value of I, that is, towards saturation, and a consequent 

 tendency in /j. to diminish towards unity. These are merely two 

 aspects of the same fact, for 



H 



H 



H 



If I has a finite limiting value and H is indefinitely increased 

 IUL must ultimately equal 1. 



These results have been confirmed by du Bois, using an entirely 

 different method depending on the Kerr effect of rotation of the 

 plane of polarisation of light reflected from a magnet pole.* 



* Phil. Mag., vol. xxix. (1890), pp. 263 and 293. 



