PROF. HUGH L. CALLENDAR AND MR. HERBERT MOSS 



.ver-Boere through holes accurately drilled in a brass plate A, which was clamped 

 .gainst the vertical face of the T joint in the tubes containing the circulating L 



Bnt.il Plai*. 



o) - 



Fig. 3. 



The mercury tubes up to the point A would thus be maintained at the temperature 

 of the circulating oil. After traversing a distance of about 5 cm. in the air, the small 

 bore tubes passed through a brass block B, being soldered into 

 holes in the block drilled so as accurately to correspond with 

 those in the plate A. The brass block B was carried by a rigid 

 bracket CD, and was cooled by a water circulation as indicated 

 in fig. 3. The plate A was adjusted so that the length of tube, 

 AB, where the temperature changed from hot to cold, should be 

 accurately horizontal. The same arrangement was adopted at 

 each of the points, E, H, C, B, fig. 1, where the mercury tubes 

 emerged from the circulating oil. The vertical heights of the 

 hot and cold columns were measured by the steel tapes h, l a , 

 fig. 1, suspended from the upper brackets, and read by levelled 

 telescopes. The effective heights of the columns were taken to 

 1* the vertical distances between the centres of the small bore 

 steel tubes, which could be measured to about 01 mm., giving 

 an order of accuracy of 1 in 20,000 in this fundamental measure- 

 ment. The steel tapes were standardized by comparison with 

 the standard invar scale, and were corrected for temperature 

 at each observation. 



The gauge tubes, as shown in figs. 1 and 4, were mounted on 

 a separate board in front of the apparatus so as to be protected 

 from vibration and screened from the radiation of the hot 

 columns. The temperature of the mercury in the gauge was 

 estimated by means of four standardized mercury thermometers 

 a, b, c, d, immersed in mercury contained in tubes of the same 

 9 gauge tubes, and placed at a distance apart equal to twice the distance 

 ing the gauge tubes. The difference of temperature between the gauge tubes 



