GH. VI.] 



DISCHARGE BY LIGHT 



65 



: Lenard's method may be described as follows: *-; 

 Light from the source L, which is a spark between 

 zinc electrodes, passes through a quartz plajbe Q, 

 where it enters the vacuum and falls upon a 

 negatively electrified aluminium plate C. B is a 

 perforated earthed screen through which the particles 

 are shot till they fall upon the plate E, which is 

 connected to an electrometer ; or if deflected by a 

 magnet they will fall upon the plate F. The amount 



FIG. 10. Lenard's apparatus for measuring the electrochemical 

 equivalent of the discharge of negative electricity from a cathode 

 illuminated by ultra-violet light in vacuo. 



IS 



of" charge received by these two terminals 

 separately measured and plotted, with magnetic 

 field as abscissa. The charge received by E tgill 

 decrease, and that received by F will increase; as 

 the magnetic field is increased, until a maximum 

 is received by F ; which will happen when the centre 

 of the plate is the middle of the stream of rays,. 

 A further increase in the magnetic force will cause 

 the charge received to fall off. The field required 

 to give this maximum is measured being determined 

 by metrical examination of the plotted curve. The 

 paths of the particles between B and F must ; be 



L.E. 



