ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 147 



APPENDIX V. 



On the Clark CelU By Dr. BZahle. 



In connection with the report of the Committee I beg to call your 

 attention to some researches I made, by order of the Physico-Technical 

 Institute at Berlin, on Clark cells. The time is too short for communi- 

 cating my measuring methods and results in full extent ; I can only give 

 you a short summary of the chief points. 



I used in my researches Lord Rayleigh's H form, the positive 

 electrode being mercury once distilled, the negative an amalgam con- 

 taining ninety parts of mercury and ten parts of zinc. The last was poured 

 into the vessel as a hot liquid, and solidified on the bottom. The paste, 

 which covers the positive electrode, is made by grinding together 

 mercurous sulphate, mercury, and a mixture of crystals and concentrated 

 solution of zinc sulphate. No heat was used in preparing this paste. 

 The mercurous sulphate was bought, and contained, according to 

 chemical analysis, no foreign ingredients. The zinc sulphate was made 

 basic by boiling with rods of metallic zinc ; after cooling, the dissolved 

 oxide of zinc precipitates, and with it the oxides of the metals more 

 negative than zinc. For oxidising the ferrous sulphate, which is always 

 present in commercial zinc sulphate, a small current was semt between 

 two platinum electrodes through the boiling basic solution ; the ferrous 

 sulphate was changed by the generated oxygen into ferric oxide, and fell 

 out. The H cells set up with these materials showed a great agreement 

 in their B.M.F.'s. I never found a difference greater than x^wo^'t of a 

 volt between the E.M.F. of any two of them. 



The next point I studied was the influence of the impurities in the 

 different materials composing the cell on the E.M.F., because on the one 

 hand it is well known that the smallest impurity of the mercury alters 

 very distinctly the E.M.F., and on the other hand the mercurous sulphate 

 I bought never contained impurities of a remarkable amount, and 

 different samples always had the same qualities ; I only investigated, as 

 the most important matter, the impurities of the zinc and its sulphate. 

 It was fon nd that the foreign ingredients of the zinc sulphate are of very 

 little importance, and that only the presence of free acid in the above- 

 described cleaning process, the result of boiling with metallic zinc, alters 

 the E.M.F. in a considerable degree. Among the impurities of the zinc 

 only those caused by metals more positive than zinc are of importance ; 

 the zinc may contain considerable quantities of the negative metals 

 without any alteration of the E.M.F. I conclude that the impurities of the 

 zinc are of greater importance. If we use it in the form of rods amalga- 

 mated on the surface, it seems to be a great advantage to dissolve the 

 zinc in mercury, using it then as a solid amalgam. 



The following are the values I found by a great number of observa- 

 tions for the temperature coefficient of different forms of cells, measuring 

 between 1"° and 30° in rising and decreasing temperature. The figures 

 here giv r, are the mean values of some cells of the same form^ treated in 

 the same inanner : — • 



' For a detailed account see ZeiUclirift fur Tnstrumentenkunde, Ajril 1892, and 

 Electrotechnischt ZeitschHft, Heft 30, 1892. 



L 2 



