488 Mr. W. B. Morton on the 



any progressive thermal phenomenon, while it possihly may 

 result in the eduction of a natural law, is very unlikely to 

 lead to anything more than the establishment of an approxi- 

 mate equation with constants characteristic only of the 

 individual materials actually employed, and not transferable 

 to other, although similar materials. Such results are obviously 

 of a much more ephemeral character than the melting-point 

 measurements. Even when any pyrometer thus tested is 

 applied to the establishment of melting-points, it must at best 

 yield results inferior to direct application of the gas ther- 

 mometer, except in cases where the latter is hampered by 

 want of sufficient quantity of the metal to be experimented 

 upon, — a condition which need only affect such costly sub- 

 stances as gold and platinum. 



Stated broadly, the great need of the art of pyrometry is 

 convenient methods of producing, or of recognizing when 

 produced, a series of accurately known high temperatures. 

 The analogous problem has been partially solved for ther- 

 mometry at temperatures up to 300° C. by the investigation 

 of boiling-points of certain chemically pure substances under 

 controlled pressures. 



Rogers Laboratory of Physics, 



Massachusetts Institute of Technology, 



Boston, September, 1895. 



LIII. Notes on the Electro-Magnetic Theory of Moving 

 Charges. By W. B. Morton, B.A* 



1. rj VEIS subject has been brought into prominence re- 

 X cently by the use which Mr. Larmor has made of 

 moving electrons in his dynamical theory of the aether. The 

 matter was investigated in 1881 by Prof. J. J. Thomson f, 

 who showed that a point charge moving so slowly that the 

 electric displacement it carries is not sensibly disturbed 

 generates magnetic force like a current element according to 

 Ampere's rule ; and by Mr. HeavisideJ, who investigated the 

 matter more generally in 1889, and showed that in steady 

 rectilinear motion at any speed less than that of light, the 

 lines of displacement continue to be radial but are concen- 

 trated towards the plane perpendicular to the direction of 

 motion. The displacement at distance r, in a direction 



* Communicated by the Physical Society: read March 27, 1896. 

 t Phil. Mag. April 1881, July 1889 • Recent Researches, pp. 16-23. 

 X Electrical Papers, ii. pp. 504-518; Electromagnetic Theory, i, 

 pp. 269-274. 



