1898.] Electrification by Ultra-violet Light. 407 



Now if P = velocity of blast, the time t x taken for the current of 

 air to pass between the electrodes is given by 



._! 



l i — p- 



Assuming that the velocity of the air blast is constant over 

 the cross section of the plates, if ^ is less than t none of the 

 charged carriers which leave C can reach the gauze B, but they 

 will be all blown out with the current of air. We may suppose 

 the carrier describes a diagonal path between the plates due to 

 the resultant of the two impressed velocities at right angles to 

 each other, and unless this diagonal path cuts the gauze B the 

 carrier will escape. 



Let A B, CD (Fig. 2) be the two plates. Suppose the carriers 

 to be produced uniformly along CD by the action of ultra-violet 

 light. A carrier starting from C travels along the diagonal path 

 CL and gives up its charge to the plate A B. Draw BP parallel 

 to LG, meeting GD at P. We see that a carrier starting from P 





Fig. 2. 



will just reach B. All carriers starting from the right of P will 

 not reach AB but will be carried out by the current, while those 

 to the left of P will give up their charges to AB. The ratio p of 

 the number of carriers blown out to the total number that leave 

 CD is given by 



PD 



P = 



CD 



the distance PD = P ,t, where t is the time taken for the carrier 

 to cross over between the plates ; 



d 2 

 :. PD = Px—, 

 uv 



p & 



I vp 



The ratio p is determined experimentally and P can be measured, 

 therefore the value of u can be at once obtained. 



