Directions for the preparation of materials used in Clark and Weston standard cells, and for tlieir construction, have been published from time to time.T he following specifications are based upon these directions and upon work done at the Bureau of Standards. They are submitted in more detail than might be considered necessary, in order to secure the fullest possible cooperation of other workers in this field in determining the accuracy of reproduction attainable. Definitions. -The Clark

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... rd cell will be understood to mean a voltaic combination having as its positive electrode pure mercury covered with a paste consisting of pure mercurous sulphate mixed with fineh' divided mercury and pure zinc sulphate crystals and solution, and as its negative electrode an amalgam containing lo per cent by weight of pure zinc, covered with a layer of pure zinc sulphate crystals, the electrolyte being a solution of pure zinc sulphate in saturation equilibrium with the other constituents of the cell. The Weston standard cell will be understood to mean a similar combination in which the zinc sulphate is replaced throughout by cadmium sulphate and the zinc amalgam by an amalgam containing 12.5 per cent by weight of pure cadmiinn.