354 Mr. J. H. J. Poole on the 



wire-gauze grid when heated by the incident light may 

 produce apparently quite large photo-electric effects, which 

 ure really quite spurious. The quartz plate, however, will 

 .manifestly not suffer from this complaint on at least three 

 accounts. These are : (a) The thermal expansion of quartz 

 is practically zero ; (b) Owing to its transparency it will 

 absorb very little energy from the incident beam ; and 

 (c) Even if the plate did expand slightly it would do so 

 in its own plane and not alter the capacity of the isolated 

 system appreciably. The chief drawback of the arrangement 

 is that it cannot obviously be used at very low pressures o wing 

 to the evaporation of the liquid film. 



The test-plate was illuminated through a quartz window 

 at the top of the- testing vessel. A quartz window was 

 • employed, and it is convenient to be able to use ultra- 

 violet light on occasion to test whether the arrangement is 

 functioning properly. As a source of light a 200-w T att 

 |-watt lamp was employed, and a suitable train of lenses 

 used to concentrate the light. A cardboard screen was 

 placed round the lamp to protect the observer's eyes, and 

 the electrometer was completely shielded from the light by 

 a large wooden screen. It was found that switching on 

 this light had a slight effect on the electrometer, although 

 the leads to the lamp were kept as far distant as possible. 

 In consequence, a shutter was used by which the illumination 

 could be controlled without turning the lamp off. 



In order to obtain some idea of the absolute current to 

 which a known rate of deflexion of the gold-leaf corresponds, 

 it is necessary to measure the capacity of the gold-leaf and 

 testing plate taken in conjunction. This capacity was deter- 

 mined by comparing it with a second capacity whose value 

 •could be calculated approximately. The method adopted 

 was that described by H. H. Dixon and H. H. Poole in their 

 paper on Photo-Synthesis (loc. citS). The capacity was found 

 to be about 6 cms., and as the sensitivity is also known we can 

 easily determine the value of the currents obtained. The value 

 obtained for the capacity is probably not very accurate, but 

 great accuracy is not requisite, as realh" all we require to 

 know in these experiments is the order of magnitude of the 

 •currents dealt with. 



The method of procedure adopted was first to spread a 

 thin film of the substance to be tried on a disk of pure lead- 

 foil, which was then placed on the testing plate of the 

 ionization chamber. The chamber was re-assembled, and 

 the pump worked till the desired pressure had been attained. 

 The rate of leak of the electrometer with the light off and on 



