iDixoN AND Poole — Photosynthesis and the Electronic Theory. 67 



very small capacity renders it suitable for measuring minute ionization 

 currents when it can be nsed alone, and not connected to external conductors ; 

 but in this case, since a conducting plate is obviously necessary on which to 

 spread the chlorophyll, the advantage of the small capacity of the electroscope 

 itself is, to a great extent, lost. It accordingly seemed to be worth while to 

 repeat the experiments, using an electrometer giving a deflexion of several 

 hundred scale divisions per volt, and magnifying any photo-electric effect by 

 working at reduced air-pressure. 



Outline of Electrometer Method. 



The usual method was adopted. The substance to be tested was placed 

 on an insulated plate p, enclosed in a testing vessel v, and connected, as 

 shov/n diagrammatically in fig. 1, to one pair of quadrants of an electrometer 

 E. The other pair of quadrants, the case of the electrometer, and the testing 

 vessel were all connected together and earthed at z. The grid G was kept 

 at a potential of about + 230 volts by means of a battery of dry cells B. 

 The key k enabled the plate and its pair of quadrants to be earthed at will. 

 The light passed through a window above the grid, and after passing through 

 the spaces in the latter fell on the plate. The difference between the ioniza- 

 tion currents with and without the light is a measure of the photo-electric 

 effect. - 



Fig. 1 , 



EXPEEIMENTAL DETAILS. 



In initial experiments an appreciable effect was produced when the light 

 was turned on. This was soon found to be spurious, and was traced to the 

 sagging of the grid under the heating effect of the light. A simple calcula- 

 tion shows that a very slight diminution in the distance between the grid 

 and the plate will cause a large effect. For example, in the last experiments 



