SEPTEsrBEE 29, 1905.] 



SCIENCE. 



393 



ELECTRODYNAMICS. 



The discovery and interpretation of 

 electr.odynaniic 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 Avith 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 nat- 

 urally preferred. 



INDUCTION. 



Faraday (1831, 1832) did not put for- 

 ward the epoch-making discovery of elec- 

 trokinetic induction in quantitative form, 

 as the great physicist was insufficiently 

 familiar with Ohm's law. Lentz, however, 

 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 mag- 

 nets and circuits. Lentz clearly announced 

 that the induced quantity is an electro- 

 motive force, independent of the diameter 

 and metal and varying, ccet. par., with the 

 number of spires. The mutual induction 

 of circuits was first carefully studied by 

 AVeber (1846), later by Filici (1852), us- 

 ing a zero method, and Faraday's self- 

 induction by Edlund (1849), while Mat- 

 teuci (1854) attested the independence of 

 induction of the interposed non-magnetic 

 medium. Plenry (1842) demonstrated the 

 successive induction of induced currents. 



Curiously enough the occurrence of eddy 

 currents in massive conductors moving in 

 the magnetic field was announced from a 

 different point of view by Arago (1824- 

 26) long before Faraday's great discovery. 

 They were but vaguely understood, how- 



ever, until Foucault (1855) made his in- 

 vestigation. The general problem of the 

 induction to be anticipated in massive con- 

 ductor is one of great interest and Helm- 

 holtz (1870), Kirchhoff (1891), Maxwell 

 (1873), Hertz (1880) and others have 

 treated it for different geometrical figures. 



The rigorous expression of the law of in- 

 duction Avas first obtained by F. Neumann 

 (1845, 1847) on the basis of Lentz 's law, 

 both for circuits and for magnets. W. 

 AA'^eber (1846) deduced the law of induc- 

 tion from his generalized law of attrac- 

 tion. 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. Fin- 

 ally Faraday's ' electrotonic state' was 

 mathematically interpreted thirty years 

 later, by Maxwell, and to-day, under the 

 name of electromagnetic momentum, it is 

 being translated into the notation of the 

 electronic theory. 



Many physicists following the funda- 

 mental equation of Neumann (1845, 1847) 

 have developed the treatment of mutual 

 and self induction with special reference 

 to experimental measurement. 



On the practical side the magneto-in- 

 ductor may be traced back to d'al Negro 

 (1832) and to Pixii (1832). The tremen- 

 dous development of induction electric ma- 

 chinery which followed the introduction of 

 Siemens 's (1857) armature can only be 

 instanced. In 1867 Siemens, improving 

 upon Wilde (1866), designed electric 

 generators without permanent magnets. 

 Pacinotti (1860) and later Gramme (1871) 

 invented the ring armature, while von 

 Hefner- A Iteneck (1872) and others im- 

 proved the drum armature. Thereafter 

 further progress was rapid. 



It took a different direction in connec- 



