291 



and the temperatnre of the bath changed until another balance was found. 

 In this way a nnmber of balances were obtained for different values of Ri. 

 By plotting Ri against the difference in temperature of the two junctions a 

 curve was obtained which gave the temperature for any resistance. The 

 calibration was made with a number of different couples and the results 

 were entirely consistent, no point being off the straight line thus found 

 more than 1-20 of a degree centigrade. 



The pressure in the bottle was measured by means of an oil manom- 

 eter ; as considerable time was consumed by the oil coming to a steady 

 state, it was deemed desirable to place a stopcock between the manometer 

 and the bottle, and after the pressure was determined cut off the mano- 

 meter before expansion. The pressure for the following trial was ad- 

 justed approximately by an auxiliary mercurial manometer and the final 

 adjustment was made with oil. The use of the oil manometer was neces- 

 sary, as the errors introduced in the reading of the mercurial manometer 

 were of a higher degree of magnitude than was permissible. 



The delicacy of the apparatus was indicated by the fact that the 

 observer could readily detect a difference in pressure of 2 mm. of oil, 

 density .Si. 



The value of y was determined as follows: — 



From the adiabatic law, P V}' = a constant 

 From the law of Charles, PV = RT 



or 



PiVi} = P2V2} 





ther 



Vvi/ P2 





but 



PiVi = RTi 





and 



P2V2 = RT2 

 V2 Pi T2 







Vi P2 Tl 



» 





(;;)'- (f! 



T2Y Pi 

 Tl/ ~ P2 



log 



Pi 

 P2 



UlU. 1 



Pi 



log — 



P2 



Tl 



— log — 

 T2 



