Xlii INTRODUCTION. 



having its anode or negative electrode of cadmium amalgam, consisting of lo 

 per cent by weight of cadmium and 90 per cent mercury. The cathode, or posi- 

 tive electrode, is pure mercury covered with a paste consisting of mercurous 

 sulphate, cadmium-sulphate crystals, and solution. The electrolyte is cadmium- 

 sulphate solution in contact with an excess of cadmium-sulphate crystals. The 

 containing vessel is of glass, usually in the H form. Connection is made to the 

 electrodes by platinum wires sealed into the glass. The cells are sealed, pref- 

 erably hermetically, and in use are submerged in a constant-temperature oil 

 bath. The resistance of a cell is about 600 to 1000 ohms. The Weston cell used 

 with potentiometers is not the Weston normal cell, but differs from it only slightly, 

 the cadmium-sulphate solution not being saturated. It is described in the next 

 section below. 



One of the great advantages of the Weston normal cell is its small change of 

 electromotive force with change of temperature. At any temperature, t (centi- 

 grade), between 0° and 40°, Et = -E20 — 0.0000406 {t — 20) — 0.00000095 (^ — 20)^ 

 -1- o.oooooooi {t — 20)^. This temperature formula was adopted by the London 

 conference of igo8. That this formula may apply, the cell must be of a strictly 

 uniform temperature throughout. One leg of the cell has a large positive and 

 the other leg a large negative temperature coefl&cient. If the temperature of 

 one leg changes faster than the other, the formula does not hold. 



When the best of care is taken as to purity of materials and mode of procedure, 

 Weston normal cells are reproducible within i part in 100,000. The source of 

 the greatest variations has probably been in the mercurous sulphate. Cells using 

 the best samples of this material have an electromotive force the constancy of 

 which over a period of one year is about i part in 100,000. Only very meager 

 specifications for the cell have as yet been agreed upon internationally, how- 

 ever, and the procedures in various laboratories differ in some respects.^ 



The basis of measurements of electromotive force is the same in all coun- 

 tries as the result of the joint international experiments of 1910. As already 

 stated, a large number of observations were made at that time with the silver 

 voltameter, and a considerable number of Weston normal cells from the na- 

 tional laboratories of England, France, Germany and the United States were 

 compared. From the results of these voltameter experiments and from resist- 

 ance measurements, the value 



1. 0183 international volts at 20° C 



was assigned to the Weston normal cell. A mean of the groups of cells from the 

 four laboratories was taken as most accurately representing the Weston normal 



^ For the preliminary specifications which have been issued and the reports of the various 

 investigations on the standard cells see the following references: Preliminary specifications, 

 Wolff and Waters, Bull. B. of S. 3, p. 623, 1907; Clark and Weston Standard Cells, Wolff and 

 Waters, ditto, 4, p. i, 1907; Temperature formula of Weston Standard Cell, ditto, 5, p. 309, 

 1908; The materials, reproducibility, etc., of the Weston Cell, Helett, Phys. Rev. 22, p. 321, 

 1906; 23, p. 166, 1906; 27, pp. 33, 337, 1908; Mercurous sulphate, etc., Steinwehr, Zs. fiir 

 Electroch. 12, p. 578, 1906; German value of cell, Jaeger and Steinwehr, ditto, 28, p. 367, 1908; 

 National Physical Laboratory researches, Smith, Phil. Trans. 207, p. 393, 1908; On the Weston 

 Cell, Haga and Boeremaj Arch. Neerland, des Sci. Exactes, 3, p. 324, 1913. 



