398 MR. F. E. SMITH ON THE NORMAL WESTON CADMIUM CELL. 



of a high-resistance (15,000 ohms) potentiometer made by O. WOLFF, of Berlin, nml 

 a Broca galvanometer of 1000 ohms resistance made in the workshops of the National 

 Physical Laboratory. It was quite easy to read to one hundred-thousandth of a volt, 

 and, if necessary, one-tenth of this could be estimated with considerable accuracy. 



When testing samples of mercurous sulphate we have often used a four-limb vessel 

 similar to two Kayleigh H-form vessels crossed. Cadmium amalgam was placed in 

 one limb, and in the other three mercury and the depolarisers were inserted. The 

 electrolyte was a saturated solution of cadmium sulphate. 



Unit of Electromotive Force. 



In a recent communication to the Royal Society, Professor AYRTON, Mr. MATHER, 

 and the author* have given the E.M.F. of some of the cells included in Table I. in 

 terms of the ampere (10" 1 C.G.S.) and the international ohm. While it must be 

 admitted that this E.M.F. is possibly different from the true E.M.F. in volts 

 (10~ 8 C.G.S.) by 2 or perhaps 3 parts in 10,000, it is probably the most accurate 

 value known, and has the further advantage of being the mean of values the 

 observations of which extended over 19 months. A further deduction is that the 

 E.M.F. of most of the cells under observation did not change in this period by more 

 than O'OOOOl volt. We have therefore given the E.M.F. of cells in Table I. in terms 

 of the ampere (10" 1 C.G.S.) and the international ohm. 



It is impossible to give all the observations over the period 1904-1907 ; those 

 given are at approximately equal intervals of time. In cases where considerable 

 changes in the E.M.F. have resulted, more extensive observations are given in 

 subsequent tables. -The values for the period May, 1904, to October, 1905, have 

 been deduced from intercomparisons of cells, as no value in terms of tl>e ampere and 

 international ohm could be assigned until the later date. 



Table I. gives the results of the observations on 60 cells. Since 1904 more than 

 200 cells have been set up, and a few of those which seem to be most valuable from 

 the point of view of results obtained are included in the table. For the 60 cells in 

 Table I., 16 samples of mercurous sulphate have been used, 6 of cadmium amalgam 

 and 5 of cadmium sulphate. In some cases the mercurous sulphate was washed with 

 alcohol ; the letter A is then inserted in column 5 of the table. The approximate 

 depth of the paste is given in column 6, and the numbers in column 7 indicate the 

 range of the dimensions of the mercurous sulphate crystals in thousandths of a 

 millimetre. Particulars of the cadmium amalgam are given in columns 8 and !) ; 

 E indicates that the amalgam was prepared by the electro-deposition of cadmium, 

 and F by the fusion of cadmium and mercury. The mercurous sulphate was usually 

 prepared two weeks in advance of its use as a depolariser in a cell. 



* W. E. ATRTON, T. MATHEK, and F. E. 8111x5, "On a New Current Weigher," 'Phil. Trans.,' 1907. 



