OS THE SILVER VOLTAMETER. .r,;; 



its prolonged contact with the atmosphere. If we exclude these results the mean is 

 ril826 g . The mean difference of 2'4 parts in 100,000 must not be taken as the 

 figure indicating the reproducibility of the Rayleigh form, for the errors of time, of 

 current determination, of weighing, and of manipulation, are factors in this, as well 

 as the ]X)ssible changes which ensue due to slight variations in anode, kathode and 

 electrolyte, and which jointly constitute the error associated with the voltameter. If 

 we exclude all of the errors except those due to the voltameter and faulty manipula- 

 tion, the mean difference is very small ; this is well illustrated in observations 

 43a, b, c, </, the results of which are given to 1 part in 1,000,000. The mean of 

 these four results is ril826 s , and the mean difference is only 7 parts in 1,000,000, 

 which is probably much lower than the usual error of manipulation. We feel 

 justified, therefore, in regarding the Rayleigh form of voltameter, as employed by us, 

 to be reproducible within 1 part in 100,000. 



The Richards Form of Voltameter. 



The results obtained with the Richards form were at first more variable and always 

 lower than when the Rayleigh form and the syphon form were employed. For some time 

 we were at a loss to understand why the Richards value should be lower than that ot 

 the syphon, for both forms do, to a considerable extent, exclude the anode liquid from 

 the kathode vessel, and the changes in concentration of the kathode liquids are also 

 comparable. We eventually found the discrepancy to be due to the porous pots, of 

 which we had three kinds : (1) From the Akron Insulator and Marble Company of 

 the United States ot America ; (2) from PUKAL of Berlin ; (3) from an unknown 

 source. Dr. GUTHE kindly obtained the pots (1) for us, and they are similar to 

 those used by him in his research on the Silver Voltameter at the National Bureau of 

 Standards.* The second type of pot is larger, but presumably of the same kind of 

 ware as the pots used by RICHARDS. The third class of pot is from an unknown 

 source ; they were made from large porous pots obtained through the agency ot 

 Messrs. W. & J. GEORGE, Ltd. 



We cleaned the pots with aqua regia, potassium cyanide, nitric acid, and hot 

 distilled water before using in the voltameter. After a few runs they became 

 stained, and further cleaning with potassium cyanide, nitric acid, and water was 

 necessary. In our earlier experiments the pots were soaked in several lots of distilled 

 water for 24 hours before using, and in neutral silver nitrate solution for 3 or more 

 hours before the erection of the voltameter. The water in which the pots were finally 

 soaked was invariably free from acid sufficient to redden very sensitive blue litmus 

 paper, but we were forced to conclude that the pots were not acid-free, for on 

 electrolysis of silver nitrate with a pot interposed between anode and kathode the 

 solution in contact with the kathode became sufficiently acid to affect litmus. An 



* GUTHE, Phys. Rev.,' 19, p. 138, 1904 ; ' Bull. Bureau of Stands.,' vol. 1, No. 1, pp. 28 and 349, 1904. 



4 C 2 



