Theory of the /Structure of the Electric Field. 301 



former one, which is rather fatiguing' to the eye, owing to 

 the intermittent flashes. Even when the speed of the shaft 

 is slow, about 300 revolutions per minute, and the flashes 

 occur at intervals of -I- second, the image of the pointer is 

 clear and well defined. I have applied this optical method 

 of reading a moving dial to a torsion work-measuring machine 

 placed between an electric motor and a dynamo feeding arc 

 lamps. The best way of reading the torsional angle of a 

 shaft is, undoubtedly, by means of a scribing point actuated 

 by differential gear Avhich makes the movement of the point 

 directly proportional to the angle of torsion. The earliest 

 application of such a device is due to Hirn (Les Pandyno- 

 mometres, par G. A. Hirn, Paris, Grauthier-Villars, 1870). 

 In it eight cog-wheels are required to move the pointer and 

 show the torsional angle. 



Subsequent inventors have recently arrived at the same 

 result with a reduction of the number of cog-wheels. In a 

 recording torsional oncometer, by the author of this com- 

 munication, four cog-wheels were employed and two flexible 

 joints. (Shown at the Royal Society, May 2, 1894.) In 

 some cases such an elaborate method of reading the torsional 

 angle is not required, and the optical method I have described 

 is sufficient for those cases in which only rather small powers 

 are dealt with, as for example, in aeroplane engines, ami 

 motor-launch internal combustion engine-. 



December 29, 1909. 



XXX. On a Theory of the Structure of the Electric Field 

 ((ml its Application to Rontgen .Radiation and to LigJit. By 

 Sir J. J. Thomson', Professor of Experimental Physics, 

 Cambridge** 



r¥*HE theory considered in this paper — that the electric field 

 _l is made up of a number of discrete units — is one which 

 naturally suggests itself, if we use the conception of tubes of 

 electric force for representing the state of the electric field. 

 I have in several papers and also in my ' Recent J Researches on 

 Electricity and Magnetism," and in * Electricity and Matter,' 

 shown how the phenomena of the electric field can be 

 regarded as the method of working of a mechanical system, 

 the parts of which are the tubes of electric force, which on 

 this view are endowed with mass, momentum, and energy. 

 The properties of the tubes of force are determined by 

 the charge at their ends, hence if that charge is the charge 



* Communicated by the Author. 



