EXPANSION OF GASES. 



51 



pressure is only allowed to vary very slightly, we may take Boyle's Law 

 as true, and therefore 2 PV/0 = constant, where we add up for each part 

 of the gas, expresses the constancy of the mass of gas in the thermo- 

 meter. In practice the pressure at the end of the experiment will not 

 be the same as at the beginning, for even if the sides of the manometer 

 are level the barometric pressure is likely to change slightly. And 

 it may be more convenient not to trouble to adjust the levels to 

 equality, but to make them nearly the same, measure the difference and 

 add it to or subtract it from the height of the barometer. 



If, then, P be the pressure when the whole apparatus is at 6 ; P' the 

 pressure when the bulb is at 0' ; if V, V be the volumes of the bulb, v, v' 

 the volumes of the connecting tube, 1ft the mean reciprocal of the tem- 

 perature of v' when the bulb is at 6', M the volume of air expelled into 

 the measuring tube when the bulb is at 0', M itself being at 0, we have 



FV' PV FM_PV Pv 



& + t + e : " e + e 



whence 



e 



P'V 



r, 



An approximate value of & will suffice to calculate V, and v' and t must 

 be estimated as exactly as possible. 



In this class of instrument, then, it is necessary to measure P and P' 

 and also M, and the cor- 

 rections for the connecting 

 tube are at least as un- 

 certain as in the other 

 class. The greater number 

 of measurements in this 

 method probably led 

 Regnault to prefer the 

 constant - volume method , 

 though good determina- 

 tions had already been 

 made of certain high 

 temperatures by Pouillet 

 with the constant-pressure 

 method. 



Callendar's Com- 

 pensating Constant-Pressure Thermometer. Callendar has devised 



a comparatively simple form of constant-pressure air thermometer, in 

 which measurements of the external pressure are no longer necessary, 

 and in which automatic corrections are made for the connecting tubes.* 

 In this instrument Y x (Fig. 38) represents the air bulb. M is the 

 measuring tube initially filled with mercury. ^ is the connecting tube. 

 V 2 is a bulb of volume equal to that of Y I} and with equal " dummy " 

 connecting tube v 2 , following as nearly as possible the course of v l and 

 closed at the end. The two systems are connected by a U tube pressure 

 gauge G, containing sulphuric acid. 



* Proc. R. S., vol. 1., 1892, p. 247. 



FiO. 38. Callendar's Compensated Air 

 Thermometer. 



