A. L. Day and J. K. Clement — Gas Thermometer. 409 



have been inherent in all gas thermometer measurements up 

 to this time. Progress is necessarily slow in work of this 

 character, but we were chiefly delayed by having to build the 

 entire equipment ah initio, except the bulb.* 



The instrument which we constructed for this work has now 

 been in operation for more than three years, but has never 

 been described in print. It is of the constant volume type, as 

 has been explained, similar in general plan to that at the 

 Eeichsanstalt. but differing from it in certain important details 

 with the especial purpose of correcting some of the known 

 errors of the Eeichsanstalt instrument : (1) A uniform temper- 

 ature along the thermometer bulb appeared to us imperative, 

 and a much greater effort was made to obtain it ; (2) the entire 

 furnace was enclosed in a gas-tight bomb in order that a nitro- 

 gen atmosphere might be maintained with equal pressures, 

 both inside and outside of the bulb. This had the effect of 

 obviating any tendency of the gas to diffuse into or out of the 

 bulb, and allowed no opportunity for the deformation of the 

 bulb through differences between the pressure within and 

 without. A further effect of this arrangement was to increase 

 the sensitiveness of the instrument fully threefold. It has 

 been the practice heretofore in such temperature measure- 

 ments to greatly reduce the initial pressure of the gas in 

 order that its final pressure at the highest temperature to be 

 measured may be substantially equal to the atmospheric pres- 

 sure without, in order that the strain on the bulb through 

 pressure difference may be least when its power to withstand 

 such strain is smallest. In the Eeichsanstalt instrument, this 

 restricts the available range of pressure for a temperature 

 range from = -1150° to about 500 mm of mercury, or less than 

 Q.gmm p er d e g ree# ;gy arranging to increase the pressure out- 

 side the bulb as the pressure within increases, this restriction 

 falls away and it is possible to extend the pressure range over 

 the whole length of the scale which the manometer carries. 

 The scale of our instrument was 1*8 meters long. For a 

 range of 1200 c , therefore, we were able to work with a sensi- 

 tiveness of a little more than l mm for each degree centigrade, 

 or rather more than three times the sensitiveness used in the 



*The bulb which was used for all the measurements here recorded was 

 one of two bulbs made by Dr. Herreus, of Hanau. Germany, for the Holborn 

 and Day investigation at the Eeichsanstalt. one of which contained 20 

 per cent iridium and the other 10 per cent, The 20 per cent iridium bulb is 

 still at the Eeichsanstalt and was used in the publications of Prof. Holborn to 

 which reference has been made. The 10 per cent iridium bulb was exhibited 

 by Dr. Heraeus at Paris, after which it was loaned to us for this investiga- 

 tion. The form and capacity of the two bulbs were substantiallv the same, 

 about 200". 



The authors take this opportunity to express their thanks to Dr. Henieus 

 for his most cordial and effective cooperation throughout this undertaking, 

 and for his personal interest in the outcome of it. 



