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



Lord RAYLEIGH* was the first to point out that the quality of the mercurous 

 sulphate used as the depolariser in Clark cells was a cause of variation in E.M.F. ; 

 and Mr. SwiNBURNEf arrived at the same conclusion in 1891. Later JAEGER and 

 LiNDECKj attributed similar variations in the Weston cadmium cell to the same salt. 



In 1902 some experiments at the National Physical Laboratory plainly indicated 

 that the mercurous sulphate might produce variations in the E.M.F. as great as 

 0'002 volt, or 1 part in 700 of the voltage of the Clark cell. A new specification of 

 the mode of manufacture of the depolariser was thought to be desirable, and 

 experiments were immediately made with this end in view. 



Almost simultaneously CARHART and HULETT,|| at the University of Michigan, and 

 WoLFF,1T at the National Bureau of Standards, Washington, attacked the same 

 problem, and a little later H. v. STEINWEHR,*^ at the Physikalisch-Technische 

 Reichsanstalt, made a special study of the change of E.M.F. produced by varying the 

 size of the crystals of the depolariser. While it must be admitted that the chemistry 

 of the standard cell is still incomplete, an analysis of results shows that different 

 observers can set up cells of almost identical E.M.F., and that their constancy is many 

 times that of the standards used ten years ago. 



PREPARATION OF THE MATERIAL. 

 Mercury. 



In all our work the commercial mercury was cleaned with dilute nitric acid, washed 

 with distilled water, and distilled twice in vacuo. 



The Depolariser. 



At the National Physical Laboratory the mercurous sulphate has been prepared in 

 four ways: (1) Electrolytically ; (2) by chemical precipitation, mercurous nitrate 

 being added to sulphuric acid ; (3) by the re-crystallisation of purchased samples of 

 mercurous sulphate from strong sulphuric acid ; and (4) by the action of fuming 

 sulphuric acid on mercury. 



The first of these methods was developed in 1904 by CARHART and HULETT|| and 

 also independently by WOLFF. If The mercurous sulphate is formed at a mercury 



* Lord RAYLEIGH, ' Phil. Trans.,' 175, p. 412, 1884, and 176, p. 781, 1886. 

 t J. SWINBURNE, ' British Association Report,' Section A, 1891. 

 t W. JAEGER and ST. LINDECK, ' Zeitschr. f. Instrumentenk.,' 21, p. 33, 1901. 

 F. E. SMITH, ' British Association Report,' Section A, 1904. 



|| H. S. CARHART and G. A. HULETT, ' Amer. Electrochem. Soc. Trans.,' 6, pp. 109-126, 1904. 

 H F. A. WOLFF, 'Amer. Electrochem. Soc. Trans.,' pp. 49-58, April 7, 1904. 



** H. v. STEINWEHR, ' Zeitschr. f. Instrumentenk.,' 25, pp. 205-208, July, 1905 ; also ' Zeitschr. f. Elek- 

 trochem.,' pp. 578-581, 190G. 



