298 ELECTRICAL MEASUREMENTS 



The zinc sulphate must be treated to remove the sulphates of 

 cadmium, iron, lead, and free sulphuric acid; the last has the 

 greatest effect on the e.m.f. and gives rise to the formation of 

 gas at the amalgam. 



The purity of the mercurous sulphate is of prime importance; 

 lack of purity of this salt is the chief cause of variation in the 

 e.m.f. The usual impurities are basic mercurous sulphate, basic 

 mercuric sulphate, traces of mercuric nitrate, etc., according to 

 the method of preparation. This salt is subject to the action 

 of light and must be prepared in subdued light and preserved 

 in the dark under dilute sulphuric acid. Great care must be 

 exercised in washing it before use to free it from sulphuric acid, 

 the washing being done with absolute alcohol, which in turn is 

 removed by zinc sulphate solution. 



This salt may be made by a number of methods, all of which 

 give satisfactory results, the best one, apparently, being an 

 electrolytic method devised by Wolff and Waters. The mercury 

 anode is at the bottom of a tall jar, the cathode a piece of plati- 

 num foil suspended above the anode, and the electrolyte is 

 dilute sulphuric acid. The mercury is violently stirred during 

 the passage of the current and for some time after the circuit is 

 broken. 



The Western or Cadmium Cell. 13 As early as 1884 Czapski 

 called attention to the low temperature coefficients of cells with 

 cadmium electrodes, but the matter was forgotten until 1892, 

 when attention was recalled to this type of cell by Edward 

 Weston. The cell suggested by him is composed of cadmium 

 in cadmium sulphate solution opposed to mercury in mercurous 

 sulphate paste. The particular advantage of this form of cell 

 is its very small temperature coefficient. This is in consequence 

 of the fact that the solubility of the cadmium salt is only slightly 

 influenced by the temperature, consequently the changes in 

 density of the solution are very small. Also the temperature 

 effects on the two limbs of the cell tend toward compensation. 

 The normal form of this cell has been studied at the Reichsan- 

 stalt by Jaegar, Kahle, Wachsmuth, and Lindeck, and in this 

 country by Wolff and Waters. The cadmium is used in the 

 form of an amalgam made by dissolving, with the aid of heat, 

 1 part of Kahlbaum's best cadmium in 7 parts of mercury. If 



