Temperature upon Photo-electric Effects in High Vacuum, 203 



which show the lower rates of discharge, while the noble 

 metals, copper, brass, silver, etc.. which in air, according to 

 Elster and Geitel*, show no photo-electro effects at all, and 

 according to Branlyt show effects whollv insignificant in 

 comparison with those of aluminium, zinc, and cadmium, 

 stand here at the very top of the series. 



In this respect our results are in agreement with those of 

 Ladenburgi, who alone, of all our predecessors in investiga- 

 tions of this sort, placed his metals in a high vacuum. Using 

 surfaces "which had been polished once with emery and oil " 

 he obtained the following series : — 



Metal. Discharge rate. 



Platinum 2*81 



Copper 2'±5 



Zinc 2-22 



Brass 2*01 



Silver 1-31 



Antimony 1*20 



Lead -80 



Bismuth *78 



Iron '(jl 



Nickel -52 



AVhiie these results bear *ome sort of resemblance to our>, 

 and are equally significant with ours in establishing the 

 complete independence of photo-electric effects in a vacuum 

 upon the Yolta contact series, the order shown is nevertheless 

 quite different from that found in Table II. The following 

 two causes are to be assigned for this difference. 



In the first place, Ladenburg found that the photo-electric 

 sensitiveness of the metals which he experimented upon 

 -howed large variation with the amount of polish of the 

 surface — a result which is of course natural, since polishin 

 with emery and oil doubtless produces a surface-film which ha 

 many properties different from that of a surface with which 

 oil has not come into contact. In order to avoid as far as 

 possible the variations due to this cause we took great care 

 in the preparation of our surfaces not to allow oil or impurities 

 of any sort to come into contact with them. The metals 

 were all polished on a wheel with dry emery, then washed in 

 alcohol and dried by heating to 400° 0. This difference in 



o 



.- 



the treatment to which our surfaces and Ladenburg's were 

 subjected may account for the fact that in our experiments 



* Elster & Geitel, Wied. Ann. xxxviii. p. 505 (1889). 

 t JBranly, Jour. d. Phys. 3rd series, ii. p. 300 (1893). 

 % Ladeuburg, Ann. d. Phys. xii. p. 558 (1903). 



