290 



necessitated a more consistent calibration. After repeated efforts to obtain 

 a constant deflection for a ghen difference in temperatni-e the method was 

 abandoned as not being snfRciently accurate. 

 The following scheme was then adopted : 



A battery of known E. M. F. was connected in series with two resist- 

 ances, R^, which was approximately 100,000 ohms, and in later experiments 

 kept constant, and Ri was varied from one to eight ohms to suit the condi- 

 tions of the particular observation. B is a carboy in the center of which 

 one junction of the couple was located. A is a second junction which was 

 kept at a constant temperature. G is a galvanometer in series with the 

 couple and Ri. The air in B Avas compressed as before and allowed to cool 

 to the temperature of the bath. Ki was then closed, then the valve was 

 opened to the atmosphere and immediately K^ was closed and the direction 

 of the deflection of the galvanometer noted. This process was repeated, 

 varying Ri until a resistance was found such that on closing K„ there was 

 no deflection of the galvanometer, until the air began to warm after the 

 adiabatic expansion. This balanced condition meant that the P. D. across 

 Ri just balanced that due to the difference in temperature of the two junc- 

 /tions of the thermo-couple. 



In practice it was found better to set Ri at a given place, e. g., 5 ohms, 

 and then vary the original pressure until a balance was obtained. In some 

 of the earlier observations R^ was varied to secure a balance, but since it 

 was not known to a sufficient degree of accuracy, the other method was 

 used. It then remained to calibrate the thermo-couple. This was done bj"- 

 placing one junction, encased in a jacket, in a constant temperature bath, 

 and the other, similarly encased, in a liatli whose temperature was varied 

 till a balance against a given resistance, Ri, was obtained. The diffei'ence 

 in temperature of the two juuctions was then noted. Ri was a^ain varied 



