Page:Proceedings of the Royal Society of London Vol 60.djvu/91

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Although the electrical resistivity of mercury at ordinary temperatures has been carefully examined by many observers, and accurate determinations made of the specific resistance* and temperature coefficient, and in addition an examination made of the variation of resistivity in mercury when cooled to temperatures as low as — 100° C.,f we considered it would be of interest to examine the behaviour of pure mercury in respect of change in electrical resistivity when cooled to the temperature obtained by the employment of boiling liquid air. With this object we prepared a sample of very pure mercury in the following manner : Ordinary distilled mercury was shaken up with nitric acid in the usual manner to free it from other metals, and then carefully dried. It was then introduced into a bent glass tube formed of hard glass. This bent tube had both ends sealed, and a side tube connected in at the bend, by which it could be connected to a mercury vacuum pump. Two or three hundred grammes of the mercury was then introduced into one bend, and a high vacuum made in the tube. The side tube was then sealed off from the pump, and the mercury distilled over from one leg into the other. For this purpose, one leg of the bent tube was placed in ice and salt, and the other submitted to a gentle heat just sufficient to make the mercury distil under reduced pressure without ever bringing it into active ebullition. In this way the mercury is distilled over at a very low temperature, and the portion condensing in the cooler limb of the bent tube is entirely free from any contamination with silver, lead, zinc, or tin. By performing this distillation two or three times successively on the same mercury, a small quantity of mercury is at last obtained in an exceedingly pure condition. A glass spiral tubo was then formed of lead glass, consisting of a tube having an internal diameter of about 2 mm., and a length of about 1 metre. This tube was bent into a spiral of about twelve close turns, each turn being nearly 2*5 cm. in diameter, and the ends of this spiral provided with enlarged glass ends formed of wider tube. The spiral,

(P hil. Trans. It. 3., Part I, 1883). See, also, Mr. R. T. Glazebrook (Phil. May., Oct., 1885), for other values. f Cailletet and Bouty ( Compt.P
 * “ The Specific Resistance of Mercury,” by Lord Rayleigh and Mrs. Sidgwick