484 Prof. A. 0. Rankine : Relation between Illumination 



Experimental Arrangement, 



The experiments were carried out in a dark room. The 

 selenium cell under test was enclosed in a light-tight box 

 provided with a photographic shutter. It formed part of a 

 circuit consisting of a 4-volt accumulator and a calibrated 

 uni-pivot microammeter of 500 ohms resistance, a key being- 

 included in order to complete the circuit at will. In no case 

 did the current exceed 10 ~ 4 ampere, so that the difference of 

 potential across the selenium did not vary from the e.m.f. of 

 the accumulator by more than ^ 6 volt. The applied voltage 

 was, in fact, constant to within 1 per cent. The source of 

 light used was a metal-filament lamp of 8-candle power 

 running on an 8-volt accumulator. It could be regarded at 

 considerable distances as a point source, since the filament 

 was only 6 mm. long. Changes of the intensity of illumi- 

 nation of the selenium were obtained by placing the lamp at 

 carefully measured different distances from the selenium 

 surface, and the relative intensities were calculated on the 

 basis of the inverse square law. The limits of distance used 

 were 4'5 metres and 0*2 metre, so that the extreme intensities 

 were in the proportion of about 500 to 1. The observations 

 were made in the order of diminishing distance or increasing 

 intensity. First of all, the selenium circuit was made for a 

 few seconds with the selenium in darkness, in order to 

 determine the "dark" current. The shutter was then 

 opened with the source 45 metres away, and the current 

 determined at intervals by using the kev. When the value 

 had become steady, the distance of the source was dimi- 

 nished and the procedure repeated. It was found that 

 three minutes exposure to each increase of illumination was 

 more than sufficient for the steady state to be attained. 

 About half an hour therefore sufficed to complete the series 

 of eleven observations — a period too short for the tempe- 

 rature of the selenium to change appreciably from chance 

 causes. 



In each case the maximum currents thus observed were 

 plotted against the fourth root of the light intensity, or the 

 reciprocal of the square root of the distance between selenium 

 and source. For all the cells examined — four in number, — 

 although they differed considerably in sensitivity to light, the 

 resulting curve was a good straight line. A typical case is 

 shown in fig. 1, on which the dotted curve is also placed to 

 show how much less exact the square-root law is than the 

 fourth-root one. The curve is represented by the equation 



C-(Wds 



