682 On a Sensitive Photo- Elect vie Cell. 



coefficient of reflexion of sodium is very high, ilie value given 

 by Drude being 9 9' 7 per cent. Hence less than 1 per cent, 

 is available for producing a photo-electric effect. 



In the new cell, however, the light which gets into it is 

 scattered about in all directions, and only a small fraction 

 escapes out of the cell. .Thus, by continued reflexion, much 

 more of the light is used up photo-electrically than when 

 only one reflexion takes place. In fact, the cell is analogous 

 to the enclosures used in radiometiy to represent a black body. 



Photo-electric cells can be used to investigate light-inten- 

 sities which are far too small to produce any effect Avith a 

 thermopile and delicate galvanometer. The sensitiveness of 

 photo-electric cells varies in different parts of the spectrum, 

 and so it would be necessary to calibrate them by means of 

 a thermopile and a source of intense light, if it is desired to 

 compare intensities in different parts of the spectrum by- 

 means of such cells. This, uf course, is unnecessary if the 

 variations of intensity in any one region are to be investi- 

 gated. Elster & Geitel * have employed photo-electric cells 

 to measure the variations of light-intensity during lunar and 

 solar eclipses. Nichols & Merritt f have applied them to 

 the study of luminescence. Since photo-electric cells can 

 measure quantity of light (intensity x time), Pichtmycr ± 

 has indicated how they could be used to measure the speed 

 of photographic shutters. Photo-electric cells can be readily 

 used for investigations in phosphorescence. 



If cells of the type described in this paper are required 

 only for wave-lengths longer than X 3300. the quartz plate 

 can be dispensed with and. the whole apparatus made of 

 glass. The winch device would be unnecessary, as the metal 

 could be driven off the part intended to act as window by 

 the application of heat. K, Hb, and Cs are still more sensi- 

 tive than Na in the visible region, and could be used instead 

 of Na if greater sensitiveness is required. This new form 

 of photo-electric cell has the advantage that only a small 

 quantity of metal is necessary, which, in the case of the 

 more expensive metals Rb and Cs, is worthy of consideration. 



I wish to express my best thanks to Professor Sir J. J. 

 Thomson for his interest in this work. 



The monochromator used for testing the cell was obtained 

 by means of a Government Grant from the Government 

 Grant Committee of the Royal Society. 



* Elster & Geitel, Pki/s. Zeits. p. 1212 (1910) ; p. 582 (1912). 

 t Nichols & Merritt, Phvs. Rev. vol. xxxiv. p. 475 (1912). 

 % Kichtmyer, Phys. Rev. vol. xxx. p. 394 (1910). 



