OAO 0B0  OcO

ODO pep OFO a b ------------------ (ti) ja~~~E -0 a WI 0 0 0 Gal, . 0 and when made as directed below it has at t° C. an electromotive force E, volts, such that E =1.0184 -0.0000406 (t -20) -0.00000095 0—20)2 + 0.00000001 (t-20)3 . After the platinum wires have been sealed through the glass, a little aqua regia is placed in the cell legs until bubbles pi gas arise from the platinum, when it is thrown out and replaced by a solution of mercurous nitrate . Then, by the use of another piece of platinum as anode, mercury is electrolytically deposited upon the platinum, which may also be amalgamated by making it white hot in a Bunsen flame and plunging it in mercury . To prepare the cadmium amalgam, one part of pure cadmium is dissolved in six parts of pure mercury, and the product while warm and fluid is placed in one limb of the cell and warmed, to ensure perfect contact with the platinum wire . The cadmium sulphate solution is prepared by digesting a saturated solution of cadmium sulphate with cadmium hydroxide to remove free acid, care being taken not to raise the temperature above 7o° C., and then by digesting it still further with mercurous sulphate until no more precipitation occurs . The cadmium sulphate solution must be saturated and have free crystals of the salt in it . The mercurous sulphate must be free from acid, and made neutral by trituration with finely divided mercury . In making the paste, so much cadmium sulphate must be added that a saturated solution of that salt is formed and is present in the cell . The cell has the electromotive force above stated if the amalgam of cadmium has from 6 to 13 parts of mercury to 1 of cadmium . The German investigators seem to have a great preference for the H form of cell, but it is clear that a narrow tubular cell of the British board of trade form not only comes more quickly to the temperature of the water bath in which it is placed, but is more certain to be wholly at one temperature . In a modification of the H form devised by F .

E .

Smith, of the National Physical Laboratory (Phil . Trans., A, 207, pp . 393-420), a contraction formed in the side of the vertical tube tends to hold the contents in place . Fig . 4 shows this cell, hermetically sealed, mounted in a brass case . In cases when great accuracy is not required, a Daniell cell can be used as a standard of electromotive force . The form designed by J . A . Fleming (Phil . Mag., 20, p . 126) consists of a U tube, one leg of which contains a rod of pure amalgamated zinc, and the other a rod of freshly electrotyped copper .

The legs are filled with solutions of zinc sulphate and copper sulphate, the zinc rod being in the zinc sulphate and the copper rod in the copper sulphate . When so made, the cell has an electromotive force of 1.072 volts and no sensible temperature variation . The solutions are made by dissolving the purest recrystallized sulphate of copper and sulphate of zinc in distilled water . For the zinc solution, take 55.5 parts by

weight of crystals of zinc sulphate (ZnSOa7OH2) and dissolve in 44.5 parts by weight of distilled water; the resulting solution should have a specific gravity of 1.200 at about 2o° C . For the sulphate of copper solution, take 16.5 parts by weight of pure crystals of copper sulphate (CuSO4,5OH2) and dissolve in 83.5 parts by weight of water; the resulting solution should have a specific gravity of 1.100 at 20° C . The solutions should be adjusted exactly to these densities and kept in stock bottles, from which the reservoirs of the cell should be filled up as required . A form of potentiometer employing a vibration galvanometer and suitable for alternating current measurement by null methods has been devised by Dr Drysdale (see Proc . Phys . Soc . Lond . 1909, 21, 561.) See J . A .

Fleming, Handbook for the

Electrical Laboratory and Testing Room, vol. i . (London, 1903)-vol. i contains on pp . 108-110 an extensive list of various original memoirs published on the Clark and Weston cells; G . D . Aspinall Parr, Electrical Engineering Measuring Instruments (London, 1903) ; W . C . Fisher, The Potentiometer and its Adjuncts (London, 1906) .