On the normal weston cadmium cell
The experimental investigations described had as their primary object the improvement of the Clark and cadmium cells as standards of electromotive force. In the past many investigators have pointed out that the mercurous sulphate used as a depolariser may produce variations in the E. M. F. as great as 0·002 volt, and the first thing sought by us was a mode of manufacture of the sulphate which could be relied on to give a constant product. We have prepared the salt in four ways: (1) Electrolytically (the method is due to Carhart and Hulett, and Wolff); (2) by chemical precipitation, mercurous nitrate being added to sulphuric acid; (3) by the recrystallisation of purchased samples of mercurous sulphate from strong sulphuric acid; and (4) by the action of fuming sulphuric acid on mercury. The mean value of the cells set up with the electrolytic salt is 1·01828 volts; with the sulphate prepared by (2) the E. M. F. is 1·01830 volts; (3) gives 1·01832 volts, and (4) gives 1·01831 volts. We conclude that the mode of manufacture of the depolariser is immaterial, provided that certain conditions are observed, and our guiding principle in the manufacture of the salt and the preparation of the paste is to prevent hydrolysis by keeping the salt in contact with dilute sulphuric acid (1 to 6) or with saturated cadmium sulphate solution.