Ratio of the Specific Heats for Air and Steam. 229 



same diameter as the thermometer wire, but of shorter length, 

 sufficiently long, however, to eliminate any end-effect error 

 due to conduction of heat from the stout platinum leads to 

 the fine platinum-thermometer wire. The four glass tubes 

 were placed closely side by side, and introduced into the 

 u air-vessel " through the stopper B. To measure the 

 resistance of the thermometer, the thermometer and com- 

 pensating leads were connected to the two arms of the 

 YVneatstone-bridge, as shown in figure 1. In order to keep 

 the heating effect in the thermometer, due to the passage 

 of the electric current, below *01° C. the current used in the 

 resistance measurements was made sufficiently small, being 

 supplied by one Leclanche-cell through 240 ohms in the 

 battery-arm. The resistances of the two ratio arms were 

 3 ohms each. To obtain the balance position, a Thomson 

 galvanometer was used, which, however, was rendered astatic 

 to avoid unsteadiness caused by magnetic disturbances. 



In all resistance measurements the galvanometer circuit 

 was kept permanently closed, the battery circuit being broken 

 or made by means of the keys. In this way trouble due to 

 thermoelectric E.M.F/s was avoided. 



Measurement of A . — Air was pumped into the " air-vessel }> 

 until the pressure inside exceeded that of the atmosphere by 

 a definite amount (about 67 cms. of oil), time being allowed 

 for the air to assume a constant temperature. The resistance 

 of the thermometer was read off by adjusting the resist- 

 ance R and the sliding contact ,9 with sufficient accuracy to 

 give the temperature of the thermometer to *01° C. The 

 battery circuit was closed by hand by means of the key K. 



Measurement of 2 . — The resistance Rwas then diminished 

 and the sliding-contact adjusted by judgment nearly to the 

 position where there would be no current through the galva- 

 nometer at the instant when the battery circuit was made 

 by means of the automatic gauge-key N. If the sliding- 

 contact was adjusted exactly to the right position, the 

 galvanometer-needle remained at rest for an instant and then 

 gradually moved off as the thermometer heated up again. 

 If, however, the shift was too small the needle gave a kick 

 in the opposite direction to that corresponding to the heating 

 up of the thermometer, came to rest, and then changed the 

 direction of its motion, and gradually moved off as before as 

 the gas heated up. In making the observations that position 

 of the slider was sought for which the kick of the galvano- 

 meter just vanished. 



To determine the time which elapsed between the instant 

 of removing the stopper !> and that at which the mercun 



