58 PHYSICS 



Magneticians have succeeded in expressing the magnetic distri- 

 bution induced in certain simple geometrical figures like the 

 sphere, the spherical shell, the ellipsoid, the infinite cylinder, the 

 ring. Green in 1828 gave an original but untrustworthy treatment 

 for the finite cylinder. Lamellar and solenoidal distributions are 

 defined by Kelvin (1850), to whom the similarity theorems (1856) 

 are also due. Kirchhoff's results for the ring were practically utilized 

 in the absolute measurements of Stoletow (1872) and of Rowland 

 (1878). 



Diamagnetism, though known since Brugmans (1778), first chal- 

 lenged the permanent interest of science in the researches of Becquerel 

 (1827) and of Faraday (1845). It is naturally included harmoniously 

 in Kelvin's great theory (1847, et seq.). Independent explanations of 

 diamagnetism, however, have by no means abandoned the field; 

 one may instance Weber's (1852) ingenious generalization of Ampere's 

 molecular currents (1820) and the broad critical deductions of Duhem 

 (1889) from the thermodynamic potential. For the treatment of 

 seolotropic magnetic media, Kelvin's (1850, 1851) theory seems to 

 be peculiarly applicable. Weber's theory would seem to lend itself 

 well to electronic treatment. 



The extremely complicated subject of magnetostriction, originally 

 observed by Matteuci (1847) and by Joule (1849) in different cases, 

 and elaborately studied by Wiedemann (1858, et seq.}, has been 

 repeatedly attacked by theoretical physicists, among whom Helm- 

 holtz (1881), Kirchhoff (1885), Boltzmann (1879), and Duhem (1891) 

 may be mentioned. None of the carefully elaborated theories accoun s 

 in detail for the facts observed. 



The relations of magnetism to light have increased in importance 

 since the fundamental discoveries of Faraday (1845) and of Verdet 

 (1854), and they have been specially enriched by the magneto-optic 

 discoveries of Kerr (1876, et seq.), of Kundt (1884, et seq.}, and more 

 recently by the Zeemann effect (1897, et seq.). Among the theorie 

 put forth for the latter, the electronic explanation of Lorentz (1898 

 1899) and that of Voigt (1899) are supplementary or at least not con 

 tradictory. The treatment of the Kerr effect has been systematized 

 by Drude (1892, 1893). The instantaneity of the rotational effect 

 was first shown by Bichat and Blondlot (1882), and this result has 

 since been found useful in chronography. Sheldon demonstrated 

 the possibility of reversing the Faraday effect. Finally terrestrial 

 magnetism was revolutionized and made accessible to absolute meas- 

 urement by Gauss (1833), and his method served Weber (1840, et 

 seq.) and his successors as a model for the definition of absolute units 

 throughout physics. Another equally important contribution from 

 the same great thinker (1840) is the elaborate treatment of the dis- 

 tribution of terrestrial magnetism, the computations of which have 



