﻿Electrical Changes induced by Ultraviolet Light. 403 



the electroscope C. The leaf used was made of aluminium. 

 This was enclosed in a metal box with mica windows, and 

 insulated by a sulphur plug S. The sulphur was surrounded 

 with an earth-connected guard-ring G. In some experiments 

 the cap of the electroscope was a fiat circular plate C (fig. 1), 

 and in others a screw-clamp X (fig. 2). The position of the 

 leaf was read by a microscope with a scale in the eyepiece. 

 Before use, the electroscope was calibrated both for negative 

 and positive charges, against a battery o£ small cells. The 

 metals examined were made into plates 6 cms. wide, 9 cms. 

 long, and about 5 mms. thick. It was found that when a 

 plate was placed so as to reflect rays on to the cap of the 

 electroscope, as indicated in fig. 1, that the electroscope was 

 discharged irrespective of the nature of its charge. The 

 circular plate C (tig. 1) was covered by an earth-connected box 

 with aluminium sides and having an open top which could be 

 covered with any material. Whether positively or negatively 

 charged, the electroscope was discharged when the top of the 

 box was covered over with either a single gold leaf, two gold 

 leaves, or with silver leaf 0*00021 millimetre thick; but with 

 very thin paper, tinfoil, or thin guttapercha no discharge could 

 be observed, whilst in the case of aluminium leaf 0*0005 mm. in 

 thickness a difference in the rate of leak was observed, but so 

 small as to make the result doubtful. The figures in Table I. 

 give the values obtained with the various screens : these 

 figures, in spite of the fact that all possible precautions were 

 taken to keep the intensity of the arc from varying, cannot 

 be correct to more than 5 per cent. 



Table I. 



Radiating 

 plate. 



Charge on 

 electroscope. 



Screen. 



Blank discharge 

 for 10 cUys. 



Discharge 

 for 10 divs. 



Zn amalgam ... 



Positive. 



Au leaf. 



130 mins. 



1 min. 15 sees. 



Zn amalgam ... 



Negative. 



Au leaf. 



130 mins. 



4 mins. 



Zn amalgam . . . 



Positive. 



Ag leaf. 



183 mins. 



16 mms. 



Zn amalgam ... 



Xegative. 



Ag leaf. 



183 mins. 



96 mins. 



Sn amalgam ... 



Positive. 



Al leaf. 



266 mins. 



210 mins. 



The rays shot off in this manner were deflectable by a 

 magnet; this was proved by allowing the radiation to pass 

 through a pair of fine slits (S) placed horizontallv, one above 



2 D2 



