228 THE DEVELOPMENT OF ELECTRICAL SCIENCE. 



Wiedemann, Mascart, etc. The greatest step in advance in recent 

 years has been the invention of the revolving-disk method by Lorenz, 

 of Copenhagen, and its subsequent improvement and application by 

 himself and by J. V. Jones. The determinations made by the latter by 

 this method are probably almost absolutely correct. 



A subject which has attracted much attention comes in incidentally 

 here, namely, the electro-magnetic theory of light propagation 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 velocity of light. In 1868 a determination of this ratio was 

 made by McKichan under Lord Kelvin's direction, and gave close agree- 

 ment with the theory. Since that time determinations have been made 

 by various methods by Maxwell, Shida, Ayrtin and Perry, J. J. 

 Thomson, Rosa, Lodge, Glazebrook, and others, with the result 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 determination 

 of standards of measurements. It is, however, one of the investigations 

 which has been made possible by the work of the British Association 

 committee in the production of instruments of precision. Prominent 

 among these instruments stands the Kelvin electrometers, and particu- 

 larly the absolute electrometer which was described in the report of the 

 British Association 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 com- 

 municated to the Royal Society by Gibson and Barclay on the specific 

 inductive capacity of paraffin. Since that time much good work has 

 been done by Boltzmau, Hopkinson, Quincke, Silow, Klemencic, Negre- 

 ano, and others. The theoretical importance of these experiments is 

 due to the fact that, according to Maxwell's theory, the specific induct- 

 ive capacity of nonmagnetic dielectrics should be proportional to the 

 squares of their indices of refraction. A wonderful verification of Max- 

 well's theory was carried 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 experiments splendid 

 illustrations of the oscillatory 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 pos 

 sible, for example, to compare the capacity of condensers by means of 

 oscillatory currents of exceedingly short periods, and thus to determiue 

 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 Rontgen and its development in photography, but 



