392 



SCIENCE. 



[N. S. Vol. XXII. No. 561. 



supplementary or at least not contradic- 

 tory. The treatment of the Kerr effect 

 has been systematized by Drude (1892, 

 1893). The instantaneity of the rota- 

 tional 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 ter- 

 restrial magnetism was revolutionized and 

 made accessible to absolute measurement 

 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 distribution of terrestrial magnet- 

 ism, the computations of which have been 

 twice modernized, in the last instance 

 by Neumeyer^ (1880). Magnetometric 

 methods have advanced but little since the 

 time of Gauss (1833), and Weber's (1853) 

 earth inductor remains a standard instru- 

 ment of research. Observationally, the de- 

 velopment of cycles of variation in the 

 earth's eongtants is looked forward to with 

 eagerness, and will probably bear on an 

 adequate theory of terrestrial magnetism, 

 yet to be framed. Arrhenius (1903) ac- 

 centuates the importance of the solar 

 cathode torrent in its bearing on the earth 's 

 magnetic phenomena. 



ELECTROM AGNETISM . 



Electromagnetism considered either in 

 theory or in its applications is, perhaps, 

 the most conspicuous creation of the nine- 

 teenth century. Beginning with Oersted's 

 great discovery of 1820, the quantitative 

 measurements of Biot and Savart (1820) 

 and Laplace 's ( 1821 ) law followed in quick 

 succession. Ampere (1820) without de- 



^Dr. L. A. Bauer kindly called my attention to 

 the more recent work of A. Schmidt, summarized 

 in Dr. Bauer's OAvn admirable paper. 



lay propounded his famous theory of mag- 

 netism. For many years the science was 

 conveniently subserved by Ampere's swim- 

 mer (1820), though his functions have 

 since advantageously yielded to Fleming's 

 hand rule for moving current elements. 

 The induction produced by ellipsoidal 

 coils or the derivative cases is fully 

 understood. In practise the rule for the 

 magnetic circuit devised by the Hopkin- 

 sons (1886) is in general use. It may be 

 regarded as a terse summary of the theories 

 of Euler (1780), Faraday, Maxwell and 

 particularly Kelvin (1872), who already 

 made explicit use of it. Nevertheless, the 

 clear-cut practical interpretation of the 

 present day had to be gradually worked 

 out by Rowland (1873, 1884), Bosanquet 

 (1883-85), Kapp (1885) and Pisati 

 (1890). 



The construction of elementary motors 

 was taken up by Faraday (1821), Ampere 

 (1822), Barlow (1822) and others, and 

 they were treated rather as laboratory 

 curiosities; for it was not until 1857 that 

 Siemens devised his shuttle wound arma- 

 ture and the development of the motor 

 thereafter went pari passu with the 

 dynamo to be presently considered. It 

 culminated in a new principle in 1888 

 when Ferraris, and somewhat later Tesla 

 (1888) and Borel (1888), introduced poly- 

 phase transmission and the more practical 

 realization of Arago's rotating magnetic 

 field (1824). 



Theoretical electromagnetics, after a 

 period of quiescence, was again enriched 

 by the discovery of the Hall effect (1879, 

 et seq.), which at once elicited wide and 

 vigorous discussion, and for which Row- 

 land (1880), Lorentz (1883), Boltzmann 

 (1886) and others put forward theories of 

 continually increasing finish. Nernst and 

 V. Ettingshausen (1886, 1887) afterwards 

 added the thermomagnetic effect. 



