PROCEEDINGS 



OF THE 



Camkitrg^ ^l^ibsnpljkal S^mtjj. 



On the Mobilities of the Ions produced in Air by Ultra-Violet 

 Light. By A. Ll. Hughes, M.Sc, 1851 Exhibition Research 

 Scholar, Scholar of Emmanuel College, Cambridge. (Communi- 

 cated by Professor Sir J. J, Thomson.) 



[Bead 9 May 1910.] 



Since the discovery of the ionisation of gases by X-rays, several 

 investigations have been made to determine whether there is any 

 increase in the conductivity of gases when exposed to ultra-violet 

 light. It is well known that a large current of negative electricity 

 can be obtained from metallic and other surfaces when ultra-violet 

 light falls upon them. Yet when the same ultra-violet light is 

 passed through a gas it is difficult to get evidence of any increased 

 conductivity. Absorption of ultra-violet light by a gas does not 

 necessarily mean a corresponding ionisation, as Henry * and 

 Whiddingtonf found for iodine vapour which absorbs ultra-violet 

 light considerably but shows no increased conductivity due to the 

 light. 



LenardJ carried out some experiments which appeared to show 

 that air was made conducting under the action of a very absorb- 

 able kind of ultra-violet light. He determined the mobilities of 

 the ions so produced, and found for the negative a mobility of 

 3"13 cm./sec. and for the positive a mobility of "0015 cm. /sec. The 

 latter is of quite a different order to the mobilities of ions pro- 

 duced by X-rays and the radiations from radio active substances. 

 This led to the suggestion that the positive carrier in Lenard's 

 experiments is not a positive ion but a dust particle which has 



* Henry, Proc. Camb. Phil. Soe. ix. p. 319, 1897. 



t Whiddington, Proc. Camb. Phil. Soc. xv. p. 189, 1909. 



X Lenard, Ann. der Physik, i. p. 486, in. p. 298, 1901. 



VOL. XV. PT, VI. 32 



