CALIBRATION OF THE PHOTOMETER 



I09 



of the particular selenium cell in the dark is therefore 

 47550 ohms. The galvanometer deflection depends on the 

 intensity of light and also on the electromotive force of the 

 voltaic cell. This latter is a single Edison cell of large 

 capacity, the electromotive force of which is 1-4 volt. As 

 a very small current is utilised for measurement, the voltage 

 remains constant day after day. Hence the deflection of 

 the galvanometer is proportional to the intensity of light. 



Fig. 32. The Portable Electric Photometer 



s, selenium cell ; d, shutter ; w, Wheatstone Bridge, with tapping 

 keys k 2> k 2 ; b, voltaic cell ; G, unipivot galvanometer. 



The galvanometer employed is of a unipivot type, which is 

 sufficiently sensitive for the purpose. 



After the adjustment of the balance, the circuits of the 

 voltaic cell and of the galvanometer are completed by 

 successive pressure on the keys K x and k 2 . The selenium 

 cell is then exposed for 5 seconds to the light to be measured, 

 in which time the deflection attains its maximum. 



Calibration of the Photometer 



In order to calibrate the instrument, Pointolite is taken 

 as the standard source of light, the small radiant of which 



