. Manual of physico-chemical measurements. um wire passing through a glass tube also projectingdownwards through the cork (Fig. 144). When the mercuroussulphate is rubbed up with the zinc sulphate, it is well to add a littlemercury to reduce any mercuric salt which may be present. Infilling the cylinder, attention must be paid to keep the upper part ofthe vessel clean, as the zinc sulphate will otherwise creep up the sides. 194 PHYSICO-CHEMICAL MEASUREMENTS CHAP. A few drops of paraffin oil poured on the zinc sulphate solutioneffectually prevents evaporation. The usual care must be devoted to
. Manual of physico-chemical measurements. um wire passing through a glass tube also projectingdownwards through the cork (Fig. 144). When the mercuroussulphate is rubbed up with the zinc sulphate, it is well to add a littlemercury to reduce any mercuric salt which may be present. Infilling the cylinder, attention must be paid to keep the upper part ofthe vessel clean, as the zinc sulphate will otherwise creep up the sides. 194 PHYSICO-CHEMICAL MEASUREMENTS CHAP. A few drops of paraffin oil poured on the zinc sulphate solutioneffectually prevents evaporation. The usual care must be devoted to the purity of the materials,although slight impurities have no marked influence.^ The zincsulphate magma should be very thick, and at least two-thirds of thezinc rod should be surrounded by crystals of the salt. The whole soon crystallises to a solid mass, so thatthe element may be inverted without themercury leaving its place. The zinc iscovered with melted sealing-wax to within1 cm. of its lower end. Such an element, when closed at 15°. through a resistance of » x 106 -„ of ^ ^ mercury, deposits 1-4379 x-^ 0-0011181 g. of silver per second, and has thus an electromotive force of 1-438 volt at 15° with respect to the legal ohm of 106 Fig. 114. cm. The value 106-3 cm. is at present supposed to be more correct, the deposition of silver through this resistance being 0-3 % smaller, and consequently also the electromotive force. The temperature coefficient is 0-0010 volt. The electromotive force of the Clark element is thus— 1 Clark= 1-4336 - 0-0010(f - 15) volt. The Clark elements recommended by the Imperial German Insti-tute at Charlottenburg are somewhat differently arranged. Theyconsist of an H-shaped vessel (first employed by Lord Rayleigh), thebottom of one side of which is covered with a layer of mercury, theother side containing an amalgam of 9 parts mercury and 1 partzinc (Fig. 145). The zinc amalgam is poured in when warm, andsolidifies on cooling. The mercury
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