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



Reich sanstalt instrument, and also to vary the initial pressure 

 considerably without serious loss of sensitiveness. (3) In the 

 capillary connecting link between the bulb and the mano- 

 meter, we were able to diminish the volume of the unheated 

 space to about one-third of its former value, and thereby still 

 further to reduce one of the classical errors of gas thermometry. 

 This "unheated space," * it will be remembered, serves to con- 

 nect the bulb which contains the expanding gas at a certain 

 temperature and pressure, with the manometer in which the 

 pressure is measured. This space is therefore filled with gas 

 which forms a part of the total gas content of the bulb, but is 

 not heated with it and therefore requires a correction the 

 magnitude of which has sometimes been so great as to create 

 misgivings about the trustworthiness of the resulting pressure 

 obtained. f The ratio of the volume of the unheated space to the 



total volume of the bulb ( — ) in the final form of the o-as ther- 



mometer used, by Holborn and Day amounted to -0046 ; in the 

 more recent instrument used by Jaquerod and Perrot, to '0180 ; 

 while in our instrument it was reduced to "0015. The entire 

 correction for the unheated space in our instrument there- 

 fore amounted to less than 5° at 1100° compared with about 

 20° in the older Reichsanstalt instrument and about 80° in 

 the instrument used by Jaquerod and Perrot. An error of 

 10 per cent in the determination of the average temperature 

 of the unheated space in our instrument will not therefore 

 affect the result more than -5° at this temperature. (4) The 

 expansion of the bulb itself was redetermined with much 

 greater care than before. 



All these are details the importance of which we have come 

 to estimate very highly, if a really accurate temperature scale 

 based upon the expansion of a gas is to be established. The 

 effect of a serious error in any one of the four particulars noted 

 upon the temperature measurement is several times greater 

 than that arising from differences in the expansion of thevari- 

 ous available gases which formed the basis of the elaborate 

 study by Jaquerod and Perrot to which reference has just been 

 made. And here, perhaps, lies the kernel of the whole matter 

 so far as it concerns the establishment of accurate fundamental 

 temperatures in a region as remote as 1000° from the funda- 

 mental fixed points. The interest of observers is easily diverted 

 to questions of general and theoretical interest, like the validity 

 of the Gay-Lussac law over great temperature ranges, while 

 experimental conditions which permit errors of considerable 



* " Espeee nuisible," " Schadlicher Eaum." 



f See in particular Jaquerod and Perrot, Arch. d. sci. phys. et. nat., 

 Geneve (4), xx. ; pp. 28, 128, 454, 506, 1905. 



