PROGRESS IN NINETEENTH CENTURY 59 



been twice modernized, in the last instance by Neumeyer l (1880). 

 Magnetometric methods have advanced but little since the time of 

 Gauss (1833), and Weber's (1853) earth inductor remains a standard 

 instrument of research. Observationally, the development of cycles 

 of variation in the earth's constants is looked forward to with eager- 

 ness, and will probably bear on an adequate theory of terrestrial 

 magnetism, yet to be framed. Arrhenius (1903) accentuates the 

 importance of the solar cathode torrent in its bearing on the earth's 

 magnetic phenomena. 



Electromagnetism 



Electromagnetism, considered either in theory or in its applica- 

 tions, is, perhaps, the most conspicuous creation of the nineteenth 

 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 

 delay propounded his famous theory of magnetism. For many years 

 the science was conveniently subserved by Ampere's swimmer (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 prac- 

 tice the rule for the magnetic circuit devised by the Hopkinsons 

 (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 armature, and the development 

 of the motor thereafter went pari passu with the dynamo, to be pre- 

 sently considered. It culminated in a new principle in 1888, when 

 Ferraris, and somewhat later Tesla (1888) and Borel (1888), intro- 

 duced polyphase 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 for- 

 ward theories of continually increasing finish. Nernst and v. Ettings- 

 hausen (1886, 1887) afterwards added the thermomagnetic effect. 



1 Dr. L. A. Bauer kindly called my attention to the more recent work of A. 

 Schmidt summarized in Dr. Bauer's own admirable paper. 



