254 Prof. J. J. Thomson on the Emission of 



negative electricity in the cathode-rays*. Quite recently 

 Mr. 0. W. Richardson f has made a series o£ measurements at 

 the Cavendish Laboratory of the rate at which the electricity 

 escapes at different temperatures. The results of these mea- 

 surements are very interesting ; they show that surprisingly 

 large currents can pass in the best vacua between a negatively 

 electrified incandescent ware and a conductor placed in its 

 neighbourhood ; thus Richardson has shown that the negative 

 electricity streams so fast from carbon at a white heat as to be 

 equivalent to a current of about 1 ampere for each square 

 centimetre of carbon surface. If we suppose that the cor- 

 puscles which carry the negative charge have the same 

 kinetic energy as the same number of molecules of a perfect 

 gas at the same temperature, this stream of corpuscles 

 would carry with them from the metal energy at the rate of 

 about y 1 ^ of a calorie per square centimetre of surface per 

 second : the number of corpuscles coming in each second 

 from this area is about 5 X 10 18 . The question naturally 

 suggests itself whether this great crowd of corpuscles does 

 not produce other effects besides the electrical ones already 

 mentioned : it is the object of this paper to indicate some of 

 these effects. 



In the first place, since the corpuscles carry a charge of 

 negative electricity, they will move when acted on by an 

 electric force; so that, assuming the Electromagnetic Theory 

 of Light, they will be set in motion by a wave of light ; they 

 will thus absorb energy from the wave and give out this 

 energy as scattered light. We can easily calculate the energy 

 in the light scattered in this way. 



The rate at which a small charged particle, charge e and 

 acceleration /, radiates energy is equal to 



le 2 / 2 

 3 V 



where V is the velocity of light. If the charged particle is 

 acted on by an electric force X, then 



- Xe 

 m ' 



where m is the mass of the particle ; hence the rate at which 

 the charged particle is emitting energy is equal to 



i_?l!x* 



3m 2 V 



* J, J. Thomson, Phil. Mag. xlviii. p. 547. 



t O. W. Richardson, Proc. Camb. Phil. Soc. xi. p. 286. 



