298 Mr. I. 0. Griffith on the Electricity set free 



plates AA and BB. Narrow slits were cut in a circular area 

 of 1*5 cms. diameter in the central part of the plate BB to 

 allow the light to pass through and fall on the plate AA. 



> TO ELECT fiO METER 



B/7/1SS TUBE LEA DIM 



TO PUMP A/VD C(/4CE 



The plate BB is glued on to the ebonite ring EE, which is 

 1*3 cms. deep and whose outer and inner radii are 3'2 and 

 1*5 cms. respectively. A quartz plate fixed to the zinc 

 plate BB and a brass disk CC cemented to the opposite side 

 of the ebonite ring, render the chamber air-tight. An ebonite 

 plug fitted into a hole in this disk served to hold firmly a 

 brass rod d to which the zinc plate AA was attached. 



AA was connected to one set of quadrants of an electro- 

 meter, BB to the positive pole of a battery of small lead 

 cells, the other pole of which was earthed, while the brass 

 disk CC was permanently earthed, and thus prevented any 

 electricity creeping over the surface of the ebonite to the 

 insulated system connected to the electrometer. 



Under the action of the light the plate AA gives off 

 negatively charged electrons, and acquires accordingly a 

 positive charge. Owing to the collisions which take place 

 between these electrons and the molecules of air in the 

 detector, positively charged ions are generated which are 

 attracted to the plate AA, and thus increase the actual effect 

 due to the light. 



