Photo-Electric Theory of Visi 



on. 



353 



the electrometer. The copper rod itself is insulated from 

 the main body of the chamber, which is earthed, by sealing- 

 wax and a quartz tube as depicted. The sealing-wax serves 

 to make the joint air-tight. About 3 mm. above the testing- 

 plate, a small quartz plate carried by a brass ring is fitted. 

 The latter is supported by a small steel rod, which passes 

 through the side of the vessel and is also insulated with a 

 glass tube and sealing-wax. This plate replaces the ordinary 

 grid of copper gauze or other form of grating usually 

 employed to collect the negative ions escaping from the 

 photo-electric surface. By applying a thin film of a fairly 

 strong solution of either H 2 S0 4 or P 2 5 to the lower surface 

 of the plate, it can be made sufficiently conducting for 

 working with the high voltages and small currents met with 

 in these experiments. The outer brass ring which supports 



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the plate also serves to make contact with the liquid film. 

 To facilitate the escape of the electrons from the test plate 3 

 the upper quartz plate is connected through its steel sup- 

 porting rod to the positive pole of a high-voltage battery. 

 At low pressures the potential gradient was high enough to 

 largely increase the electronic current by ionization due 

 to collision, but in experiments carried out at air-pressuie 

 this would not be the case. 



The use of the moistened quartz plate, instead of the usual 

 form of wire grid, has some great advantages. The most 

 obvious one lies in its superior transparency, but it also 

 possesses a great virtue in that its shape is much more 

 constant than that of the wire grid. The importance of 

 this fact has been pointed out by H. H. Dixon and H. H. Poole 

 in a paper on " Photo-Synthesis and the Electronic Theory " 

 (Proc. Roy. Dublin Soc. vol. xvi. N. 8. No. 5), in which they 

 show that the change in capacity due to the sagging o£ a 



