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Determination of the Ratio of Specific Heats of Dry Air. 



E. K. Chapman. 



The following method for determining the ratio of specific heats was 

 suggested by some work in connection with an experiment in a fog chamber. 

 It became necessary to know the temperature in a fog chamber on sudden 

 expansion and consequent condensation of vapor. In order to measure this 

 temperature a thermo-couple of Cu and Fe was introduced, and the deflec- 

 tion of a galvanometer connected in series with the couple was noted on 

 the expansion of the saturated vapor. The couple was then graduated by 

 keeping one junction at a constant temperature and noting the deflection 

 of the galvanometer for a given change in temperature of the other junc- 

 tion. Knowing, then, the constant of the apparatus, the temperature in the 

 fog chamber was easily determined. 



The attempt was then made to use this method for finding the tem- 

 perature in a chamber of air on sudden expansion, and thus determine the 

 ratio of the specific heats. 



To the stopper of a glass carboy was fitted a large valve that could 

 readily be opened or closed by hand. One junction of the thermo-couple 

 was introduced into the carboy through a rubber stopper fitted in a hole 

 drilled in the side. The inner ends of the bent tube carrying the couple 

 were then separated by twisting the tubes in the rubber stopper. The other 

 junction was encased in a small glass bulb just outside the bottle and this 

 kept at a temperature of the surrounding medium. Later in the work 

 the entire apparatus, excepting the valve, was immersed in a bath which 

 could be maintained at a constant temperature. Dry air was then pumped 

 into the bottle and the whole was allowed to stand until it had regained 

 the temperature of the surroundings. On opening the valve the temperature 

 falls, due to the adiabatic expansion, and the galvanometer is deflected 

 because of the difference in temperature of the two junctions of the couple. 

 From this deflection it was hoped that tlie lowest temperature in the cham- 

 ber might be calculated. A great deal of difficulty was experienced in try- 

 ing to calibrate the couple, since the deflections due to a given difference 

 |u temperature varied considerably, and the degrees of accuracy desired 



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