76 Profs. J. Dewar and J. A. Fleming. On the Electrical 



" On the Electrical Resistivity of Pure Mercury at the Tem- 

 perature of Liquid Air." By JAMES DEWAR, LL.D., F.R.S., 

 Fullerian Professor of Chemistry in the Royal Institution, 

 and J. A. FLEMING, M.A., D.Sc., F.R.S., Professor of 

 Electrical Engineering in University College, London. 

 Received May 19, Read June 4, 1896. 



Although the electrical resistivity of mercury at ordinary tem- 

 peratures has been carefully examined by many observers, and accu- 

 rate 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 resist- 

 ivity 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 con- 

 nected 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 suc- 

 cessively on the same mercury, a small quantity of mercury is at last 

 obtained in an exceedingly pure condition. A glass spiral tube 

 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, 



* " The Specific Resistance of Mercury," by Lord Rayleigli and Mrs. Sidgwick 

 (Phil. Trans. R. S., Part I, 1883). See, also, Mr. E. T. GUazebrook (Phil. Mag., 

 Oct., 1885), for other values. 



f Cailletet and Bouty (Compt. Rend., 100, 1188, 1885). 



