PROGRESS IN NINETEENTH CENTURY 61 



Faraday's " electrotonic state " was mathematically interpreted 

 thirty years later, by Maxwell, and to-day, under the name of elec- 

 tromagnetic momentum, it is being translated into the notation of 

 the electronic theory. 



Many physicists, following the fundamental equation of Neumann 

 (1845, 1847), have developed the treatment of mutual and self in- 

 duction with special reference to experimental measurement. 



On the practical side the magneto-inductor may be traced back 

 to d'al Negro (1832) and to Pixii (1832). The tremendous devel- 

 opment of induction electric machinery which followed the intro- 

 duction of Siemens's (1857) armature can only be instanced. In 1867 

 Siemens, improving upon Wilde (1866), designed electric generators 

 without permanent magnets. Pacinotti '(18(50) and later Gramme 

 (1871) invented the ring armature, while von Hefner-Alteneck (1872) 

 and others improved the drum armature. Thereafter further progress 

 was rapid. 



It took a different direction in connection with the Ferraris (1888) 

 motor by the development of the induction coil of the laboratory 

 (Faraday, 1831; Neef, 1839; Ruhmkoff, 1853) into the transformer 

 (Gaulard and Gibbs, 1882-84) of the arts. Among special apparatus 

 Hughes (1879) contributed the induction balance, and Tesla (1891) 

 the high frequency transformer. The Elihu Thompson effect (1887) 

 has also been variously used. 



In 1860 Reiss devised a telephone, in a form, however, not at once 

 capable of practical development. Bell in 1875 invented a different 

 instrument which needed only the microphone (1878) of Hughes 

 and others to introduce it permanently into the arts. Of particu- 

 lar importance in its bearing on telegraphy, long associated with 

 the names of Gauss and Weber (1833) or practically with Morse 

 and Vail (1837), is the theory of conduction with distributed capac- 

 ity and inductance established by Kelvin (1856) and extended by 

 Kirchhoff (1857). The working success of the Atlantic cable demon- 

 strated the acumen of the guiding physicist. 



Electric Oscillation 



The subject of electric oscillation announced in a remarkable paper 

 of Henry in 1842 and threshed out in its main features by Kelvin; 

 in 1856, followed by Kirchhoff's treatment of the transmission of 

 oscillations along a wire (1857), has become of discriminating im- 

 portance between Maxwell's theory of the electric field and the 

 other equally profound theories of an earlier date. These crucial 

 experiments contributed by Hertz (1887, et seq.) showed that elec- 

 tromagnetic waves move with the velocity of light, and like it 

 are capable of being reflected, refracted, brought to interference, and 



