CH. VL] DISCHARGE BY LIGHT 59 



according to the direction of its lines of force. This 

 phenomenon suggested a magnetic deflexion of the 

 lines of leak, which were shown by Kighi to be 

 singularly definite trajectories, and indicated that 

 the leakage was due to the bodily propulsion of 

 negatively electrified particles analogous to the 

 cathode rays. A vacuum is not necessary to observe 

 the effect, but in a vacuum the effect is more pro- 

 minent and more accurately measurable. The 

 difference between this case and an ordinary vacuum- 

 tube case, is that there is no great E.M.F. or gradient 

 of potential applied, so there is nothing of the 

 nature of a disruptive discharge ; in fact there is no 

 leak at all until by the stimulus of the presumably 

 synchronous vibrations of ultra-violet light the 

 molecules are thrown into a state of agitation, and 

 the attachment of the negative charge, or of some 

 negatively charged corpuscles, thereby loosened. 



Two things are necessary to get the particles away 

 from the plate ; they must be loosened by the impact 

 of ultra-violet light the direction of polarisation of 

 this light having a very decided influence when 

 the surface is smooth, and the surface on which 

 they exist must likewise be negatively charged, 

 so as to repel them. Neither light alone nor 

 electrification alone will produce any considerable 

 effect ; co-operation is necessary. Light alone is 

 able to cause a slight positive electrification,* by 

 the diffusion away of negative corpuscles a process 

 which is assisted by a blast of air. Whether 

 electrification alone can produce a perceptible effect 

 depends on the temperature of the metal : when 

 that is high, it can as was discovered by Guthrie. 



*This effect was discovered simultaneously by Righi and by 

 Hallwachs. See Phil. Mag., 1888, April, p. 314, and July, p. 78. 



