Constant Pressure Gas Thermometer 



297 



Fur. 1. 



constant pressure law is often evaded by a combination of 

 the constant volume law with Boyle's law. Apart altogether 

 from the advisability of a direct verification, more parti- 

 cularly as this is the form in which the law is most 

 frequently applied, the latter method is quite unsuitable 

 for pupils at the age when this is generally taken up in 

 schools. 



The instrument described below has surmounted those 

 difficulties, so far as ordinary laboratory work is concerned, 

 and may also be used where even a very considerable degree 

 of accuracy is required. 



The gas thermometer consists of a glass bulb of about 

 150 c.c. capacity connected by a siphon S to a graduated 

 tube. During an experiment there is always in the bulb 

 from 10 to 50 c.c. of mercury which siphons over into the 

 measuring tube. In this way the expansion of the gas is 

 made to take place wholly within the bulb, and therefore 

 within the heater. As the gas expands more mercury is 

 expelled and its volume measured. 



A capillary tube D leaves the 

 top of the bulb and joins the 

 siphon-tube lower down, forming 

 a level indicator outside the heater 

 and enabling the pressure to be 

 accurately adjusted. There is never 

 more than one thousandth of the 

 whole volume of the gas outside 

 the heater. 



By first filling the bulb com- 

 pletely with mercury from the top 

 of the measuring tube and packing 

 the heater with ice, 100 c.c. of any 

 gas may be allowed to enter the 

 bulb at a temperature of 0° C. The 

 volume may be measured at any 

 temperature between 0° and 100° 0. 

 The calculation of the coefficient of 

 expansion of the gas is greatly 

 simplified by taking 100 c.c. at 0° C. 

 and heating through a range of 

 100 degrees. The expansion of the 

 flask is to a large extent com- 

 pensated by the expansion of the 

 mercury, and the error particularly at 100° C. is almost 

 negligible. 



Phil Mag. S. 6. Vol. 20. No. 116. Aug. 1910. X 



