from a Zinc Plate hy Ultra- Violet Light. 



301 



A description of the procedure adopted in the case of two 

 distances will show how this was secured. 



In fig. 3, S denotes the spark, D the detector, Q x Q 2 two 

 quartz plates placed so as to allow the light to pass through 



Fiff. 3. 



VACUUM 



t 



QtQi 



•><- 



TO PUMP 



•S 



>s 



them normally. The spark was allowed to run for 10 seconds 

 and the effect n : noted. D S was then increased from r to 

 r + d. and a tube of length d was inserted between D and S. 

 This tube, when the quartz plates are fixed on to its ends, 

 can be rapidly exhausted until the pressure of the residual 

 air in it is less than 1 mm. 



The spark was again run for 10 seconds, and the effect n 2 

 noted. 



The above process was repeated, n x and n 2 being measured 

 alternately and the mean of several readings taken as the 

 final result. 



The value of r was never less than 10 cms., so that the 

 maximum error in reading the distances could not exceed 

 1 per cent. 



By using tubes of different lengths a range of intensity of 

 1 : 190 was obtained. 



In order to work with weaker intensity and avoid the 

 inconvenience of very long distances, a quartz cell containing 

 water was placed between the spark and the detector. 



Results. 



I denotes the intensity of the light in arbitrary units. 



q is a factor which gives the effect in amperes per square 



centimetre of surface. 

 r is the distance between 

 centimetres. 



the spark and the detector in 



