Cady — Measurement of an Air Thermometer Bulb. 341 



Akt. XLIX. — Volume Measurement of an Air 

 mometer BuXb ; by Walter G. Cady. 



Ther- 



In all constant volume air thermometry where high tempera- 

 tures are involved, it is frequently necessary to determine the 

 volume of the glass bulb used ; such a bulb softens at a low red 

 heat sufficiently to have its volume considerably altered by any 

 difference between the atmospheric pressure and that within 

 the bulb. Thus it is important to know at the end of an 

 experiment to what extent, if any, the volume of the bulb may 

 have changed, as any such change must be taken into account 

 in calculating the temperature. 



The usual method of volumetry by weighing with water is 

 long and arduous, involving as it does the taking apart of the 

 apparatus. Below is given a simple and sufficiently accurate 

 method of calculating the volume at any time without discon- 

 necting the bulb. In addition to the usual apparatus it is only 

 necessary that the manometer tube in which the air is confined 

 he graduated for 9 or 10^°" from the top, so that the volume of 

 air in the tube may be computed. 



In the figure, B is the bulb, PP 

 a capillary connecting tube, T the 

 graduated manometer tube, the grad- 

 uated portion of which is about 1^"" 

 in diameter, connected in the cus- 

 tomary manner by flexible rubber 

 tubing with a reservoir. The scale 

 in question is shown at T and is 

 graduated in O'l'^"^* beginning with 

 the fiducial mark. 



Assuming the whole apparatus to 

 be at constant temperature during 

 the operation, the required volume 

 Fis easily found as 



P.^. 



P.v, 



~ P —P 



when v^ and v^ are volume excesses measured by means of 

 the scale on T, corresponding to pressures P^ and P^ respec- 

 tively. 



It will be observed that F is not the volume of the bulb 

 alone. Still, since the bulb is the only variable factor, the above 

 formula indicates what change it may undergo. 



The following is an example of a number of observations 

 leading to the volume of a glass bulb used ; in each case the 



