C. A. Perkins — Magnetic Permeability of Nickel. 219 



All the formulas introduce certain constants depending on 

 the relation between the magnetizing force and the magnetism 

 produced, to find the value of which, as well as to test the 

 theories themselves, experiments have been made repeatedly. 



The first series of experiments undertaken in a really scien- 

 tific manner are those made by Lentz and Jacobi.* 



The method employed by them was to place a bar of iron in 

 a helix of wire and to pass through this a current whose 

 strength was measured by the attraction between two spirals 

 through which it passed. The magnetization was measured by 

 the induced current produced in a coil surrounding the iron bar 

 and the magnetizing spiral. In this way they arrived at the 

 result that the total magnetization was proportional to the 

 strength of the current. 



Joulef discovered, in experimenting on the strength of elec- 

 tromagnets in 1839 and 1840, that he could not indefinitely in- 

 crease their carrying power by an increase in the current, but 

 that it approached a maximum as the iron became "saturated." 

 He also clearly stated the law that the maximum is inde- 

 pendent of the length of the magnet and varies directly with 

 the smallest cross-section. 



But although Joule's experiments established the fact that 

 the magnetization of iron was not proportional to the current 

 strength, yet the principle was already stated in a paper by 

 Eitchie^: in 1833, six years before the publication of Lentz and 

 Jacobi's results. 



He made use of two horse-shoe electromagnets of very 

 different lengths and an armature, all of the same metal. Then 

 putting a different number of coils of wire about each magnet, 

 he connected both at the same time with a battery and found 

 the shorter one had twice the power of the longer. Then 

 keeping the connections the same he joined a larger battery and 

 found that the long magnet had increased in proportion much 

 more than the short one. He states that this is because the 

 particles in the short magnet were arranged by the first current 

 in the position to give the best effect, so that the stronger one 

 could not add much to it. 



Pliicker§ found in 1848 that bodies which at a distance were 

 attracted by a magnet were repelled when brought very close. 

 This was true of charcoal and a number of organic substances. 

 This was ascribed by him to the different rate of increase in 

 magnetization as the magnetizing force increased, and proved 

 not only that the magnetization was not proportional to the 

 force but that it might be entirely reversed. 



*Pogg. Ann., vol. xlvii, 1839. 



f Phil. Mag., [4], vol. ii, 1851. Annals of Elec, vol. iv, 1839. 



i Phil. Trans., vol. cxxiii. §Pogg. Ann., 1848. 



