Maech 25, 1898.] 



SCIENCE. 



403 



has been the invention of the revolving disc 

 method by Lorenz, of Copenhagen, and its 

 subsequent improvement and application 

 by himself and by J. V. Jones. The de- 

 terminations made by the laltter by this 

 method are probably almost absolutely cor- 

 rect. 



A subject which has attracted much at- 

 tention comes in incidentally here, namely, 

 the electro-magnetic theory of light propaga- 

 tion suggested by Maxwell. According to 

 this theory the ratio of the electro-magnetic 

 unit of quantity of electricity to the electro- 

 static unit ought to be the same as the ve- 

 locity of light. In 1868 a determination of 

 this ratio was made by McKichan under 

 Lord Kelvin's direction, and gave close 

 agreement with the theory. Since that 

 time determinations have been made by 

 various methods by Maxwell, Shida, 

 Ayrtin & Perry, J. J. Thomson, Eosa, 

 Lodge, Glazebrook and others, with the re- 

 sult that the ratio of the two units does not 

 differ from the velocity of light by more than 

 the probable error of observation. The 

 work here referred to may not appear to be 

 very^ directly associated with the determi- 

 nation of standards of measurements. It 

 is, however, one of the investigations which 

 has been made possible by the work of the 

 B. A. committee in the production of in- 

 struments of precision. Prominent among 

 these instruments stands the Kelvin elec- 

 trometers, and particularly the absolute elec- 

 trometer which was described in the report 

 of the B. A. committee for 1867. 



Another subject of great interest in itself 

 and in connection with Maxwell's theory is 

 that of the specific inductive capacity of 

 dielectrics. Experiments on this subject 

 were made by Faraday, but comparatively 

 little was done before 1870, in which year 

 an excellent paper was communicated to 

 the Eoyal Society, by Gibson and Barclaj^, 

 on the specific inductive capacity of parafiBn. 

 Since that time much good work has been 



done by Boltzman, Hopkinson, Quincke, 

 Silow, Klemencic, Negreano and others. 

 The theoretical importance of these ex- 

 periments is due to the fact, that, accord- 

 ing to Maxwell's theory, the specific induc- 

 tive capacity of non-magnetic dielectrics 

 should be proportioned to the squares of 

 their indices of refraction. A wonderful 

 verification of Maxwell's theory was car- 

 ried out only some ten years ago by Hertz, 

 who showed not only that electrical waves 

 exist, but also how to measure their wave 

 length and period. We have in these ex- 

 periments splendid illustrations of the os- 

 cillatory discharge referred to above, as 

 discovered by Henry and predicted by 

 Thomson, and as a result several new ways 

 of determining electrical quantities have 

 been developed. It is now possible, for 

 example, to compare the capacity of con- 

 densers by means of oscillatory currents of 

 exceedingly short periods, and thus to de- 

 termine the dielectric constants of many 

 materials to which the older methods were 

 not easily applicable. 



It is somewhat difficult to decide where 

 to place a reference to the recent discovery 

 of Eontgen and its development in photog- 

 raphy, but probably it comes in well here. 

 Just how to apply Maxwell's equation to 

 Eontgen rays is not yet quite clear, but 

 there is no doubt as to the great impor- 

 tance of the discovery. 



As an outcome of all this activity in the 

 determination of standards and in the ab- 

 solute measurements of the electrical prop- 

 erties of materials, combined with the great 

 commercial demand produced by the intro- 

 duction of dynamo-machinery, we have now 

 many excellent instruments at our disposal 

 for absolute measurement and suitable 

 either for practical applications or for the 

 most refined laboratory work. For the 

 production of these we are indebted to a 

 host of inventors, prominent among whom 

 may be mentioned Lord Kelvin, Lord Eay- 



