EXPANSION OF GASES. 



M' 



into connection with the manometer or closing that connection and 



putting the manometer into connection with the outside air. When the 



tap is turned on in the latter way, mercury 



is poured into M' until it oozes out at R. 



R is then turned so as to put the bulb in 



communication with the manometer, and the 



instrument is ready for use. 



Hydrogen Thermometer. For a long 



time the normal air thermometer gave the 

 generally accepted scale of temperature. 

 Since 1887, however, it has been superseded 

 by the scale of a hydrogen thermometer at 

 the Bureau International. This instrument 

 is also on the constant- volume principle. The 

 bulb is a cylinder of platinum-iridium 110 cm. 

 long, and with about 1 litre capacity. It is 

 placed in a horizontal position in a vessel 

 with a window, so that mercury thermometers 

 may be laid alongside, and their readings 

 compared with those of the hydrogen thermo- 

 meter. -The open side of the manometer is 

 used as the cistern of a barometer. The 

 difference in level between the mercury at 

 the top of the barometer and that in the 

 manometer where the gas presses on it is the 

 total pressure. Thus the separate reading 

 of the barometer is avoided. The pressure 

 is adjusted to be 1 metre of mercury at 

 0C. 



The temperature on the gas scale is 

 obtained from the observations with a gas 

 thermometer in the following way. For the 

 small variations of volume allowed, Boyle's 

 Law is sufficiently exact, and, therefore, for a given mass of gas : 



Pressure x volume/temperature from gas zero = constant. 



The fact that the mass of the gas in the thermometer is constant is 

 therefore expressed by 



2 PV/0 = constant, 



PV 



where we form the quantity s - for each part of the volume, and add the 



t> 



results. Let V be the volume of the bulb at 6, V its volume at 0'. Let v 

 be the volume of the connecting tube to the manometer at 6, v' its volume 

 while the temperature & is being measured. The temperature of v will- 

 be different at different points. Let - be the mean of the reciprocals of 



c 



these temperatures. Let P and P' be the observed pressures. 

 Then. P'V PV_PV Pp 



~~~"~~'' = ~~ ~~ 



FIG. 37. Simple Air 

 Thermometer. 



