Comparisons of Mercury and Platinum Thermometers. 31 

 Constants of Tonnelot 11801. 



Tonnelot's 11800, 11801, and 11811 were made of French 

 " verre-dur," with transparent stems divided into tenths of a 

 degree, about December 1895. They were subsequently sent 

 to the Bureau International, where a most exhaustive study 

 of them was made by M. Guillaume, and comparisons made 

 at several different temperatures with their standards. These 

 standards have been compared by M. Chappuis . with the 

 standard nitrogen and hydrogen thermometer (Trav. et Mem. 

 die Bar. Int. des Poids et Mesures, torn. vi. 1888), so that the 

 scale of these thermometers can at once be reduced to the 

 nitrogen or hydrogen scale. 



We can only briefly outline the methods used at the Bureau 

 International for the standardization of thermometers ; for to 

 do full justice to their beautiful and painstaking researches 

 on thermometry would not lie within the limits of the present 

 communication. 



The thermometers are first roughly examined for uniformity 

 of bore and graduation. If these are not sufficiently accurate, 

 the thermometer is rejected. If these are found satisfactory, 

 a calibration is next made by measuring the lengths of suitable 

 columns of mercury in different portions of the scale. This 

 determines the correction that must be applied to certain 

 " principal points " of the scale, to reduce the scale-reading 

 to what the reading would have been if the bore had been per- 

 fectly uniform. In these thermometers, whose range includes 

 0° and 100°, the corrections are usually so calculated that the 

 calibration corrections for 0° and 100° are zero. 



The next constants determined are the external and internal 

 pressure-coefficients, usually denoted by j3 e and /3;. The ex- 

 ternal pressure-coefficient is determined by subjecting the 

 thermometer to known variations in pressure and observing 

 the corresponding change in the stem-reading. The external 

 pressure-coefficient, /3 e , is then the change in degrees produced 

 by a change in the external pressure of 1 mm. of mercury. 

 The internal pressure-coefficient, j3i, is deduced from (3 e by adding 

 O- 0000154, a quantity depending on the difference of com- 

 pressibility of mercury and " verre-dur." 



The fundamental interval, i. e., the number of scale-divisions 

 between 0° and 100°, is then determined by observing the 

 " fixed points " (in steam and ice) of the thermometer. This 

 gives the value of the degree or the scale-division (if the 

 oraduation is arbitrary) on the centigrade scale. 



From these known constants the temperature on the 

 centigrade scale of this particular thermometer can be de- 



