Clark and Weston standard cells . ow into the limb, to adepth of to 2 cm, except in the cells first set up, care being takento keep the walls of the cell clean and to avoid trapping air bubbles. Introduction of the crystals and solution.—The amalgam andpaste were next covered to a depth of about 1 cm with saturatedzinc sulphate or cadmium sulphate solution, after which pulverizedzinc sulphate or cadmium sulphate was introduced—a little at atime to avoid trapping air bubbles—to a depth of to 2 cm bymeans of a wide-stemmed funnel. The cell was then filled slightlyabove the cross arm with
Clark and Weston standard cells . ow into the limb, to adepth of to 2 cm, except in the cells first set up, care being takento keep the walls of the cell clean and to avoid trapping air bubbles. Introduction of the crystals and solution.—The amalgam andpaste were next covered to a depth of about 1 cm with saturatedzinc sulphate or cadmium sulphate solution, after which pulverizedzinc sulphate or cadmium sulphate was introduced—a little at atime to avoid trapping air bubbles—to a depth of to 2 cm bymeans of a wide-stemmed funnel. The cell was then filled slightlyabove the cross arm with saturated zinc sulphate or cadmium sul-phate solution. In order to maintain a concentration equilibrium at all tempera-tures to which the cell was likely to be exposed, a saturated solutionof zinc or cadmium sulphate and a sufficient excess of the corre-sponding crystals over the amalgam and in the paste were layer of crystals in the mercury limb was intended as a further iull. Bur. of Standards Vol. 4, No. 1,. Fig. 5.—Finished Cells showing Traveling Contacts and Method of Mounting. #5/5*1 Clark and Weston Standard Cells. 33 precaution against their leaching out of the paste, which may happenwhen the solution is not completely saturated. —The cell was next hermetically sealed by the aid oftwo small horizontal blow-pipe flames applied to the cell wall fromopposite directions. One limb of the cell was closed by a cork andthe other by a cork nicked at one side, through which passed a glassrod to serve as a handle in drawing it out. The cell was graduallyheated 2 to 3 cm above the level of the liquid, until the danger ofcracking had passed, and then held in the flame, rotating mean-while, until the tube almost collapsed, after which it was closed bydrawing it out slowly while still in the flame. By judicious heatingthe seal could be nicely rounded by the expansion of the inclosedair. The second limb was sealed in a similar manner. The finishedcel
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