54 PHYSICS 



bearing on the character of electric force was discovered by Far- 

 aday in 1837. Of the theories propounded to account for this pro- 

 perty the most far-reaching is Maxwell's (1865), which culminates in 

 the unique result showing that the refraction index of a medium 

 is the square root of its specific inductive capacity. With regard 

 to Maxwell's theory of the Faraday stress in the ether as compared 

 with the subsequent development of electrostriction in other media 

 by many authors, notably by Boltzmann (1880) and by Kirchhoff 

 (1885), it is observable that the tendency of the former to assign 

 concrete physical properties to the tube of force is growing, partic- 

 ularly in connection with radioactivity. Duhem (1892, 1895) in- 

 sists, however, on the greater trustworthiness of the thermodynamic 

 potential. 



The seemingly trivial subject of pyroelectricity interpreted by 

 ^Epinus (1756) and studied by Brewster (1825), has none the less 

 elicited much discussion and curiosity, a vast number of data by 

 Hankel (1839-93) and others, and a succinct explanation by Kelvin 

 (1860, 1878). Similarly piezoelectricity, discovered by the brothers 

 Curie (1880), has been made the subject of a searching investigation 

 by Voigt (1890). Finally Kerr (1875, et seq.) observed the occurrence 

 of double refraction in an electrically polarized medium. Recent 

 researches, among which those of Lemoine (1896) are most accurate, 

 have determined the phase difference corresponding to the Kerr 

 effect under normal conditions, while Voigt (1899) has adduced 

 an adequate theory. 



Certain electrostatic inventions have had a marked bearing on the 

 development of electricity. We may mention in particular Kelvin's 

 quadrant electrometer (1867) and Lippmann's capillary electrometer 

 (1873). Moreover, among apparatus originating in Nicholson's dupli- 

 cator (1788) and Volta's electrophorus, the Topler-Holtz machine 

 (1865-67), with the recent improvement due to Wimshurst, has 

 replaced all others. Atmospheric electricity, after the memorable 

 experiment of Franklin (1751), made little progress until Kelvin 

 (1860) organized a systematic attack. More recently a revival of 

 interest began with Exner (1886), but more particularly with Linss 

 (1887), who insisted on the fundamental importance of a detailed 

 knowledge of atmospheric conduction. It is in this direction that the 

 recent vigorous treatment of the atmosphere as an ionized medium 

 has progressed, owing chiefly to the indefatigable devotion of Elster 

 and Geitel (1899, et seq.) and of C. T. R. Wilson (1897, et seq.). Quali- 

 tatively the main phenomena of atmospheric electricity are now 

 plausibly accounted for; quantitatively there is as yet very little 

 specific information. 



