CHAPTER XIV 



THE ELECTRIC PHOTOMETER 



The Portable Electric Photometer — Calibration of the Photometer — 

 Measurement of widely varying intensities of light — Best aspect of 

 sky for uniform light — Variation of light from northern sky in the 

 course of the day — Sunlight and artificial light. 



Having found that the intensity of light is nearly uniform 

 from ii a.m. to i p.m., we have next to choose the most 

 suitable position for utilising the outdoor light for the 

 experiments. For certain special investigations sunlight 

 would be required ; but such an intense light, if used for 

 too long a time, is liable to induce physiological depression. 

 The most suitable source of illumination for general purposes 

 is therefore the light from the sky. The plants are at their 

 best after the rainy season, but the sky then becomes 

 clouded in an unexpected manner ; one has therefore to 

 take advantage of the short periods during which the sky is 

 clear. In the winter season, from November to January, 

 the days are generally unclouded, but the physiological con- 

 dition of the plant is not at its best on account of the cold. 

 In investigations carried out at different seasons of the year, 

 the important condition of constancy of light has to be kept 

 in view ; the intensity of light during the course of the 

 experiment has therefore to be repeatedly measured. 



The apparatus devised for this purpose is a Portable 

 Electric Photometer, which has been rendered highly sensi- 

 tive and reliable (fig. 32). Selenium is also used here as the 

 sensitive element ; it is placed in the fourth arm of a small 

 Wheatstone Bridge of a box pattern. The ratio of the 

 arms of the bridge is 100 : 1000. Balance was secured in 

 the dark when the third arm was 4755 ohms ; the resistance 



