60 PHYSICS 



Electrodynamics 



The discovery and interpretation of electrodynamic phenomena 

 were the burden of the unique researches of Ampere (1820, et seq., 

 Memoir, 1826). Not until 1846, however, were Ampere's results 

 critically tested. This examination came with great originality from 

 Weber using the bifilar dynamometer of his own invention. Grass- 

 mann (1845), Maxwell (1873), and others have invented elementary 

 laws differing from Ampere's; but as Stefan (1869) showed that an 

 indefinite number of such laws might be constructed to meet the 

 given integral conditions, the original law is naturally preferred. 



Induction 



Faraday (1831, 1832) did not put forward the epoch-making dis- 

 covery of electrokinetic induction in quantitative form, as the great 

 physicist was insufficiently familiar with Ohm's law. Lentz, how- 

 ever, soon supplied the requisite interpretation in a series of papers 

 (1833, 1835) which contain his well-known law both for the mutual 

 inductions of circuits and of magnets and circuits. Lentz clearly 

 announced that the induced quantity is an electromotive force, in- 

 dependent of the diameter and metal and varying, caeteris paribus, 

 with the number of spires. The mutual induction of circuits was 

 first carefully studied by Weber (1846), later by Filici (1852), using 

 a zero method, and Faraday's self-induction by Edlund (1849), 

 while Matteuci (1854) attested the independence of induction of the 

 interposed non-magnetic medium. Henry (1842) demonstrated the 

 successive induction of induced currents. 



Curiously enough the occurrence of eddy currents in massive con- 

 ductors moving in the magnetic field was announced from a differ- 

 ent point of view by Arago (1824-26) long before Faraday's great 

 discovery. They were but vaguely understood, however, until Fou- 

 cault (1855) made his investigation. The general problem of the 

 induction to be anticipated in massive conductor is one of great 

 interest, and Helmholtz (1870), Kirchhoff (1891), Maxwell (1873), 

 Hertz (1880), and others have treated it for different geometrical 

 figures. 



The rigorous expression of the law of induction was first ob- 

 tained by F. Neumann (1845, 1847) on the basis of Lentz 's law, both 

 for circuits and for magnets. W. Weber (1846) deduced the law of 

 induction from his generalized law of attraction. More acceptably, 

 however, Helmholtz (1847), and shortly after him Kelvin (1848), 

 showed the law of induction to be a necessary consequence of the 

 law of the conservation of energy, of Ohm's and Joule's law. In 

 1851 Helmholtz treated the induction in branched circuits. Finally 



