ON THE ARSOLUTE EXPANSION OF MERCURY. 3 



as possible, to give the same order of accuracy in the absolute expansion that is 

 obtainable in the relative expansion by the weight thermometer method. 



2. General Description of the Apparatus and Method. 



The origin and progress of the present investigation has already been briefly 

 sketched in two previous notes (CALLENDAR, " Note on the Boiling- Point of Sulphur," 

 ' Roy. Soc. Proc.,' A, vol. 81, p. 363, and CALLENDAR and Moss, " The Boiling-Point 

 of Sulphur corrected by reference to New Observations on the Absolute Expansion of 

 Mercury," ' Roy. Soc. Proc.,' A, vol. 83, p. 106, 1909), and need not be repeated here. 

 The general arrangement of the apparatus will readily be gathered from fig. 1, which 

 shows a front and end elevation, and also a plan partly in section. 



In place of the single pair of hot and cold columns, each 1'5 m. long, employed by 

 REGNAULT, six pairs of hot and cold columns, each nearly 2 m. long, were connected 

 in series, giving nearly eight times the expansion obtainable with REGNAULT'S 

 apparatus. The connections of the multiple manometer are indicated diagram- 

 matically in fig. 2a. The hot and cold columns are marked H and C respectively, 

 and occur alternately. If the mercury when in equilibrium stands at a in the gauge 

 tube connected to the first cold column, and at z in the gauge tube connected to the 

 last hot column, the difference of level to be measured, represented by i?, will be six 

 times that due to a single pair of hot and cold columns. In the actual apparatus the 

 cross tube ef was doubled back, so that fg lay behind be, and ih behind ed, and so on, 

 giving the arrangement represented in fig. 2b. All the hot columns were placed 

 together in one limb EH (fig. 1) of a rectangle EFGH of iron tube, 5 cm.. in bore, 

 filled with circulating oil, and lagged with asbestos. All the cold columns were 

 located in the corresponding limb BC of the similar rectangle ABCD. The outer 

 limbs of the rectangles were utilized for the electrical heating coils FG, and the ice 

 cooling bath WXYZ respectively. Centrifugal circulators continuously driven by an 

 electric motor were provided for maintaining the oil in rapid circulation through the 

 rectangles, so that the temperatures of the hot and cold columns were each nearly 

 uniform. This arrangement of electric heating contributed greatly to the efficiency 

 of the apparatus, as it produced the least possible disturbance of the surrounding 

 conditions, and permitted the most easy and accurate regulation of the temperature. 



The mean temperatures of the hot and cold columns were observed by means of a 

 pair of platinum thermometers t t and t a , contained in tubes similar in size to the 

 tubes containing the mercury columns. The lengths of the loops of platinum wire 

 forming the bulbs of the thermometers were made as nearly as possible equal to the 

 lengths of the columns, and were fixed at the same level in the tubes, so as to give 

 the true mean temperature, in case there were any appreciable variation throughout 

 the length of the column. 



The free ends of the series of hot and cold columns were connected, as indicated in 

 fig. 1, by thick- walled rubber tubing to the glass tubes of the gauge, which is shown 



B 2 



