Constant Pressure Gas Ihermometer 



299 



standardized thermometer was used and a correction applied 

 for that part of the mercury thread which projected outside 

 the heater. Also when the stop-cock on the top of the bulb 

 is left open so that the pressure inside the bulb is exactly 

 equal to that outside, it is found that the mercury in the 

 level indicator suffers a small capillary depression as compared 

 with that in the measuring tube ; this amounted to exactly 

 one of the smallest divisions on the graduated tube. By 

 bringing the level indicator in front of the graduated tube 

 an allowance is made for this in each reading. 



By means of a preliminary experiment the errors of 

 volume due to the expansion of the flask, of the mercury, 

 and to the unequal heating of the mercury outside the 

 heater, etc. were determined as follows : — 



The bulb was completely filled with clean dry mercury 

 at 0° C. and heated to a temperature of 100° 0. The readings 

 on the graduated tube at the intermediate temperatures give 

 the expansion of the mercury in addition to the other errors 

 mentioned above. To obtain the error in a gas- expansion 

 experiment at any temperature t, let v be the total volume 

 of gas within the bulb (obtained by adding the expansion 

 found at temperature t to the initial volume at 0° C.) then the 

 expansion at temperature t of this volume of mercury can be 

 accurately determined by calculation from the known ex- 

 pansion of mercury. If we subtract this calculated expansion 

 of a volume v of mercury from the reading in the preliminary 

 experiment at temperature t, then the difference gives us 

 the error of the instrument at that temperature. By a 

 preliminary experiment of this kind the instrument can be 

 standardized. The following are the errors thus obtained for 

 the apparatus used in the succeeding investigations : — 



Temperature. 



0° c. ... 



Final correction 

 (to be subtracted). 

 c.c. 



10° c. ... 



*14 c.c. 



20° C. . . . 



-18 c.c. 



30° C. ... 



-23 c.c. 



40° C. ... 



-21 c.c. 



50° 0. ... 



-20 c.c. 



60° C. ... 



-18 c.c. 



70° C. ... 



-18 c.c. 



80° C. ... 



'16 c.c. 



90° C 



100° C. ... 



-12 c.c. 



-07 c.c. 



It will be seen that in every case the error is very smal 



X2 



