ON THE ABSOLUTE EXPANSION OF MERCURY. 5 



on a larger scale in fig. 4 (p. 6). The glass tubes of the gaxige were 1'5 cm. in bore, 

 and were fixed on either side of a standard invar metre of H section, to which the 

 difference of level was directly referred by means of a pair of levelled telescopes, 

 fitted with micrometer eye-pieces, and turning about a long vertical axis. The 

 readings could be taken to O'OOl cm. The difference of level was about 20'5 cm. for 

 a difference of temperature of 100 C., permitting an order of accuracy of 1 in 20,000. 

 As indicated roughly in fig. 2b, the length of the hot column l a was in general 

 greater than the length of the cold column h, owing to the expansion of the iron 

 tube rectangles containing the circulating oil. The expansion amounted to about 



1 H 



_r> a 3 



f n <: H 



I 



\t J 



* c 



Fig. 2a. 



i. 26. 



8 mm. for 300 C., and necessitated a flexible connection between the hot and cold 

 columns at the upper ends between the points e and b. The iron tube rectangles were 

 firmly supported at the base, so that the lower cross tube cd was always very nearly 

 horizontal. We are here concerned with the linear expansion only of the containing 

 tubes, which will not affect the accuracy of the absolute values of the expansion of 

 mercury, provided that adequate means are adopted for measuring the actual lengths 

 of the hot and cold columns at each observation. The provision made by REGNAULT 

 for this purpose was unsatisfactory, as he himself points out, especially in relation to 

 his fourth series of observations, for which his apparatus was not originally designed. 

 The essential point is that the tubes containing the mercury should be of small bore, 

 and should be maintained accurately horizontal at the points where they emerge from 

 the oil bath, and where the temperature changes from hot to cold. The method 

 adopted for securing this result in the present investigation is shown in fig. 3. Steel 

 tubes of 1 mm. bore were brazed with pure copper, using borax as a flux, into the 

 upper and lower ends of the vertical steel tubes, 3'5 mm. bore, containing the mercury 

 columns. The small bore tubes were bent round through a right angle, and were 



