MEASUREMENT OF POTENTIAL DIFFERENCE 297 



much more permanent than the Board of Trade form and has 

 a much smaller lag error only about one-fourth as great. Local 

 actions arising from differences in density are also avoided. 



The form of cell mentioned in the specifications prepared by 

 the National Academy of Sciences in compliance with the Act 

 of Congress dated July 12, 1894, was developed at the Reich- 

 sanstalt, by Kahle, and is shown in Fig. 171. 



The value of the e.m.f. at 15C. stated in the Act of 1894 is 

 1.434, but subsequent investigation has shown that the value 

 really is 1.4328 international volts. 



The H form of Clark cell has the disadvantage of short life, 

 for the glass is likely to crack where the platinum electrode is 

 fused in at the amalgam terminal. 



Materials Used in Standard Cells. In order that the e.m.f. of 

 any form of primary standard cell may accord with the stated 

 value, great care must be exercised in the preparation of the 

 materials, the processes for which have been carefully worked 

 out by Kahle and later by Wolff and Waters. 13 



Many of the impurities ordinarily found in the chemicals 

 employed have but a small effect on the e.m.f., so that secondary 

 standards may be set up with the best of c.p. materials, except 

 the mercurous sulphate, which must be specially prepared. Cells 

 so set up should not differ by more than 0.01 per cent, from those 

 where the greatest care has been exercised in the preparation of 

 the materials. 



The mercury, if badly contaminated, may be subjected to a 

 preliminary purification by electrolysis. The mercury is made 

 the anode, a piece of platinum foil the cathode, and the electro- 

 lyte is 2 per cent, nitric acid in water; the mercury is constantly 

 stirred. The more positive metals go almost completely into 

 solution by electrolysis; the less positive metals, which affect the 

 e.m.f. but little, are left in the mercury, which is then distilled 

 twice in a current of air at greatly reduced pressure. This 

 oxidizes the remaining impurities which distil with the mercury; 

 the oxides float on the surface and are removed by passing 

 the mercury through a pinhole in a filter paper. 



The zinc is distilled at reduced pressure to remove the small 

 amounts of cadmium, lead, iron, and arsenic, which are the usual 

 impurities. 



