Electrons emitted in Photo-Electric Effects. 103 



stand for two days, after which the alloy was finally trans- 

 ferred to the bottom of the wide tube where it was to be 

 illuminated. 



Fiff. 3. 



3rd*bulb\ 



The light was given by a mercury lamp. A Wratten 

 blue filter was used to limit the light to the wave-lengths 

 X4360 and X4050, where the selective effect is near its 

 maximum for sodium-potassium alloy. The image of a 

 small aperture in a sheet of metal close to the mercury lamp 

 was focussed on the surface of the alloy just under the 

 electrode E. The image formed a spot of light about 4 mm. 

 square, which did not shift more than 1 mm. on rotating the 

 Nicoi prism. The amount of light received by the alloy 

 could be altered by varying the aperture of the lens without 

 altering the dimensions of the patch of light on the surface 

 of the alloy. 



The charge received by the electrode E was measured by 

 a null method. The plate of a condenser of capacity 250 cm. 

 was connected to the same quadrant of an electrometer as E. 

 The potential on the other plate was altered at such a rate 

 as to keep the needle of the electrometer undetected. If 

 the potential on the condenser has to be altered at the rate 

 of V volts in a time t, the photo-electric current received 

 by E is 



. cv 



where C is the capacity of the condenser. 



