220 C. A. Perkins — Magnetic Permeability of Nickel. 



Three years later, Faraday* in attempting to classify magnetic 

 substances: glass, water, etc., found that the order was different 

 at different distances from the magnet, showing that the increase 

 of magnetization was not directly as the magnetizing force and 

 that the law was different for different materials. 



In 1849 Miillerf undertook an extensive series of experiments 

 fully establishing the fact that the magnetization of iron was a 

 complicated function of the magnetizing force and proposed the 

 equation I=220<i 2 tan ^nnnrg-c? 2 as sufficiently representing the 

 experiments. His method consisted in observing the deflection 

 of a needle by a bar of iron magnetized by a belix through 

 which a measured current was passed. 



In 1852 W. Weber^: stated his theory of induced magnetism 

 and proposed an equation based upon the same. He also made 

 experiments to find the connection between the theory and 

 observation. The method employed by him was the deflec- 

 tion of a needle by a long thin rod closely surrounded by a 

 spiral which extended also beyond the rod. The magnetizing 

 current was measured and its force calculated so that the whole 

 might be reduced to absolute measure. He found a fair agree- 

 ment with the theory and determined the maximum magnetiza- 

 tion possible for iron. 



Beetz§ in 1860 showed most conclusively that iron had a 

 maximum magnetization and he perhaps approached it as nearly 

 as is possible. A fine line was made on a varnished silver wire 

 and iron deposited electrolytically while under the influence of 

 a magnetizing force and thus the molecules were so arranged 

 as to give the greatest effect. In this way a high degree of 

 permanent magnetization was given to the filament of pure iron 

 and a stronger magnetizing force only served to slightly increase 

 the temporary magnetization. 



More recent experiments! have showed that the law of varia- 

 tion proportional to the current was not true even for weak 

 currents, the increase being at first more rapid, then less so, 

 than this law would allow. This was most noticeable in the 

 experiments of Quintus Iciliusl" to which attention was forcibly 

 called by Stoletow in a paper which appeared in Pogg. Ann.,, 

 vol. cxlvi. Instead of a cylindrical bar he used a very long 

 ellipsoid, thus avoiding the error of distribution neglected by 

 his predecessors. The magnetism was measured by the direct 

 action on a needle or by the induced current produced in a helix 

 about the inducing spiral. 



Stoletow's paper contains valuable discussions of previous ex- 

 periments and important results obtained by himself. The 



*Exp. Ees., vol. iii, p. 503, 1851. f Pogg. Ann., vol. lxxix. 



\ Pogg. Ann., vol. Ixxxvii. § Pogg. Ann., vol. ciii. 



|| Wiedemann Galvanismus, vol. ii, p. 350. ^ Pogg. Ann., vol. cxxi. 



