14 Dr. W. S. Day on a Comparison of 



5. Pressure Coefficients. 



a. Exterior Pressure Coefficients. 



In measuring the exterior pressure- cc efficient, precautions 

 were taken, following Guillaume, to have the thermometer - 

 bulb practically surrounded by a large mass of water which 

 was at a temperature several degrees cooler than that of the 

 room. The water in this grew warmer continuously, and 

 with great uniformity. This prevented errors due to a falling- 

 meniscus, as readings were not taken immediately after the 

 pressure and the reading of the thermometer were reduced. 

 Readings were taken alternately at atmospheric pressure and 

 at a pressure of 5-10 centim. of mercury. The average of 

 the pair at the high pressure of one atmosphere was compared 

 with the intervening reading at a low pressure. Ten or 

 twenty observations of this kind were made for each thermo- 

 meter and the results averaged. In this way the pressure- 

 coefficient in centimetres on the stem was found. As the 

 bores of the thermometer-tubes were not very uniform in 

 diameter, this was reduced to degrees centigrade by means 

 of Rowland's tables. 



The observations are given in Table IV. p. 15. 

 The results are as follows : — 



For Baudin 6163, /3 e =0°'000274, 

 For Baudin 6165, £,=0 a 000296, 

 For Baudin 6166, &=0°'000318. 



b. Interior Pressure Coefficients. 



In getting the interior pressure-coefficient, the International 

 Bureau, as mentioned before, calculates it from the external 

 pressure-coefficient. When the thermometer is made of 

 French hard glass, the internal coefficient is connected with 

 the external by the equation 



ft = ft + 0°- 0000154, 

 where the unit of measurement is degrees centigrade per 

 millimetre of mercury pressure. Part of this difference 

 depends on the cubical compression of mercury, and part on 

 the elastic constants of the glass. The Baudin thermometers 

 were not made of French hard glass, but it is probable that 

 this difference would not be significantly changed by this 

 fact. The same value has therefore been used. This gives 

 for the three thermometers the following values for the 

 internal pressure-coefficients : — 



For Baudin 6163, #=0°-0U0290, 

 For Baudin 6165, # = 0°"000311, 

 For Baudin 6166, &=0 o '000334. 



