248-250] Light produced by Electrical Action 209 



In the former case, if light can be caused at all by chemical action 

 independently of electrical action, the direct cause of radiation is pre- 

 sumably the vibratory agitation of the atoms produced by the disruption 

 of molecules. 



Our knowledge of light emitted by a gas on account of electrical action, 

 as for instance the well-known light of the vacuum tube, is at once more satis- 

 factory and more definite. The cause of radiation seems to be the vibrations 

 of the atoms caused by their being bombarded by free corpuscles. 



Let us apply the considerations of this chapter to the collisions which cause 

 this radiation. The mass of a corpuscle is, to within a few per cent., equal 

 to about Tj-fa of that of the hydrogen atom. Hence if we assume, as a pre- 

 liminary hypothesis, that the corpuscles take up velocities in their motion 

 equal to those which would be possessed by the molecules of a gas of masses 

 equal to those of the free corpuscles, then the value of C, as will be seen 

 from the table on p. 113, which corresponds to a temperature of 100 C., 

 will be about 7 x 10 6 cm. per sec. The ratio of this velocity to the radius 

 to an atom is 7 x 10 14 , a ratio which is therefore comparable with, although 

 not equal to, the frequency of light vibrations. 



In many cases, however, the velocity of the corpuscles will be due to the 

 action of an electric field. For instance in a vacuum tube in which the 

 gas is hydrogen at a pressure of 2 mm., the free path (cf. 8) is of the order 

 of magnitude of T ^ mm. If we assume a fall of potential of 40 volts per cm., 

 the fall on the free path is ^ volt, or 25 x 10 6 C.G.S. (electromagnetic) units. 

 The velocity v at collision is therefore given by 



|mv 2 = 25 x 10 6 x e, 



and on taking e/m = 10 7 , we obtain v = 2'2 x 10 7 . The ratio of this to the 

 radius of the hydrogen atom is 2'2 xlO 18 , a number sufficiently large to be 

 comparable with the frequency of even violet light. 



In this way we see that the emission of light as the result of electric 

 action is possible either when the free path is large, as in a vacuum tube, or 

 when the electric intensity is great, as in the electric arc. 



It therefore appears that our theory would almost enable us to predict 

 the emission of light which is produced by electrical action : at any rate 

 its consequences are fully in accordance with this observed emission. 



j. 



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