Methods and apparatus. 



25 



Part of an actual calibration will serve to show the nature of the results 

 obtained. After the preliminary adjustment of the current the following 

 data were taken: First, the temperature in the flask, then as quickly as 

 possible the potentiometer readings for the junctions A, B, C, D, E, F, and G. 

 It might be remarked that the potentiometer setting was kept fixed throughout 

 and the deflections noted; this saved a great deal of time. These were 

 quickly followed by a second reading of the temperature in the flask, and also 

 a reading of the temperature in the constant oven. Finally the stray electro- 

 motive force was measured and the current balanced as before. Then the 

 series of observations was immediately repeated. The mercury thermometers 

 were read to 0.002 C, and the deflections to the nearest half millimeter. 

 The data are given in table 2. 



Table 2. Calibration of thermal junctions, February 1, 1911. 



'Not used in computations. The figure given is the reading of the variable resistance when the current is balanced. 

 COMPUTATIONS FOR THE CALIBRATION. 



The corrected potentiometer balance differs from the setting in two respects : 

 (1) correction for stray electromotive force must be made, and (2) since the 

 balance consists partly of a setting and partly of a deflection, the effect of 

 this deflection must be included. 



The correction for stray electromotive force is obtained in direction and 

 amount by consideration of the following facts: The galvanometer connec- 

 tions are such that a positive deflection requires a decrease in the potentiom- 

 eter setting to bring it to 0; therefore, a positive stray deflection, such as is 

 seen in table 2, by demanding a further decrease in the potentiometer setting, 

 which is already 0, indicates a stray electromotive force opposite in direction 

 to the net electromotive force of the thermal-junction system. Hence, the 



