50 



HEAT. 



whence 



& 



P'Y' 





An approximate value of & will suffice to determine V from V. 

 Unless the temperature & be very high, it is not necessary to take into 

 account the increase of volume through increase of internal pressure, 

 and the expansion, with rise of temperature, may be determined by 

 preliminary experiments. The greatest uncertainty is introduced by the 



term , for it is difficult to determine t. 



Jrt 



Calendar's and Bottomley's Constant-Volume Thermometers. 



It is evidently advis- 

 able to diminish v as 

 much as possible ; at 

 the same time it is 

 advisable to^ have the 

 mercury manometer 

 as far removed as 

 possible from the 

 high temperature en- 

 closure. To reconcile 

 these two opposing 

 conditions, Callendar* 

 and Bottomley f have 

 both devised air ther- 

 mometers in which a 

 U tube pressure gauge 

 M containing sulphuric 

 acid is interposed be- 

 tween the air bulb and 

 the manometer. 



Bottomley's Air 

 B /T Thermometer. Fig. 



37A represents the 

 form used by Bottom- 



FlG. 37A. Bottomley's Air Thermometer. A, Air bulb ; ^' ^\ s w ^ su ^" 

 B, Sulphuric acid gauge ; P, Air pump to make pres- ciently illustrate the 

 sure equal on the two sides of B ; M, Manometer. - principle. 



The sulphuric acid 



gauge is at B, as near the bulb A as is convenient. Between this and 

 the manometer M is a force pump, P, by which air can be forced in 

 to make the two sides of the gauge B level. The manometer M then 

 indicates the pressure. The volume v or v' is that of the connecting tube 

 from the bulb to B, and the length of tubing from B to M is immaterial, 

 so that M may be quite protected from any high temperature. The 

 enlargements in B are to allow the air to expand while the temperature is 

 rising without driving the sulphuric acid out of the manometer. 



Constant-Pressure Gas Thermometers. We may take Fig. 36 



to represent the ordinary form of this class of thermometer. Since the 

 * Phil. Trans., A., 1887, p. 166. t Phil. Mag., xxvi., 1888, p. 149. 



