A. L. Day and J. K. Clement — Gas Thermometer. 425 



On account of the unequal expansion coefficients of the 

 (brass) scale, and of the (glass) manometer tube, in which the 

 fixed point is mounted, the elevation of the fixed point with 

 respect to the scale will change with temperature. The vari- 

 ation amounts to — # 0009 cm per degree rise in temperature. 



Yolume of Bulb. — The volume of the bulb, including the 

 stem, was determined by weighing with water at the beginning 

 of these experiments and again at their conclusion with the 

 following results : 



Yolume of bulb and stem, Sept. 1905 _ . . 1 95*79 cc 



" " " " Feb. 1908 195-66 cc 



Since Y enters into the computation of temperature only as 

 a part of the correction factor for the unheated space, and as 

 this total correction is never more than 5°, it is obvious that 

 the absolute volume of the bulb is not, of itself, an important 

 factor in the problem. On the other hand, the correction for 

 the expansion of the bulb with the temperature amounts to 45° 

 at 1100°, and is the most important correction factor which 

 requires to be determined. An error of 1 per cent in the 

 determination of this constant (/3) produces an error of 0*5° 

 at 1100°. 



Expansion Coefficient of the Bulb* — The determination of 

 the expansion coefficient of the bulb did not prove to be the 

 perfunctory operation -which had been anticipated, but devel- 

 oped into an independent research of somewhat exasperating 

 character, covering several months. 



There are two methods which might be pursued to obtain this 

 constant. It is theoretically possible to determine the actual 

 volume expansion of the gas thermometer bulb in position in 

 the furnace, but an effort to carry it out experimentally a few 

 years ago developed serious difficulties where the range of tem- 

 perature is so great and the accuracy required so considerable. 

 We therefore preferred to obtain a bar made from the same 

 material as the bulb, and to determine its linear expansion 

 under conditions which were under more perfect control. 



In principle, the method of procedure is the one used at the 

 Reichsanstalt. A bar of platin -iridium 5 mm in diameter and 

 slightly more than 25 cm in length was prepared for the 

 purpose and heated in a tube furnace in which the temper- 

 ature could be maintained nearly uniform from one end of the 

 bar to the other and conveniently regulated up to 1000° or 

 more. The ends of the bar were filed flat for a distance of 6 mm 

 and upon these flat surfaces millimeter divisions were ruled 

 with a dividing engine. The balance of the apparatus con- 

 sisted of a pair of micrometer telescopes mounted so as to 

 *By A. L. Day and E. B. Sosman. 



