MOLECULAR MOTIONS AND TEMPERATURE 169 



intervening portion of the stem divided into 100 equal 

 divisions. Such a thermometer is not, however, as 

 satisfactory for use as that shown in Fig. 15. In this 

 form 1 the volume of the contained gas is kept constant. 

 As the temperature changes corresponding 

 changes occur in the pressure exerted by the 

 gas and hence in the pressure required to 

 maintain its volume constant. The volume 

 is controlled by the mercury in the flexible 

 tube and the pressure exerted on the gas is 

 dependent upon the difference hi levels of the 

 mercury hi the glass tubes forming the exten- 

 sions of the flexible U tube. 



To calibrate this thermometer the bulb is 

 inserted in distilled water containing chipped 

 ice. The mercury column is then adjusted 

 so that the gas has a definite volume when 

 it has reached a temperature equilibrium 

 with the melting ice. Under these condi- 

 tions let the pressure be denoted by P . The 

 ice bath is then replaced by a steam bath 

 and the pressure, PI OO , which is required to 

 reduce the volume to V, is noted. The change 

 hi pressure which has been observed cor- 

 responds to 100 Centigrade. 



The change of pressure per degree Centigrade is 

 then (Pioo Po)/100. The fractional increase in pres- 



1 Such a piece of apparatus is used as an instrument of precision 

 for calibrating the more convenient mercury and alcohol thermom- 

 eters with which we are all familiar. The latter require calibration 

 because equal intervals on the stem do not correspond exactly to 

 equal increments in temperature. 



