MEASUREMENT OF POTENTIAL DIFFERENCE 209 



quired voltage divide the required voltage by the correction 

 factor 



0.3926 

 ~ 273 + /' 



b is the barometric pressure in millimeters and t the temperature 

 in degrees C. A new voltage is thus obtained. The spacing 

 corresponding to this new voltage as obtained from the table 

 is that required. 



The voltage at which a given gap sparks over is found by 

 taking the voltage corresponding to the spacing from the table 

 and multiplying by the above correction factor. 



POTENTIOMETER ARRANGEMENTS 



Poggendorf Method of Comparing a Potential Difference 

 and an Electromotive Force. All of the various methods now 

 used for the rapid standardization of 



f 1 E,\ B, 



direct-current instruments depend on 

 the ability to compare a potential 

 difference with an e.m.f. This may 

 l>e accomplished by Poggendorf 's 

 method, which is shown diagram- 

 inatically in Fig. 153. E\ and Ez are 

 the e.m.f s. of the batteries at EI and 

 Ez. The cell at EI will of necessity 

 have the higher e.m.f. /u and Rz are 

 two variable resistances, K a key 

 which is normally open, and G a suit- 

 able galvanometer. The circuit of EI is closed through the re- 

 sistance RI + Rz, across which there will be established a poten- 

 tial difference, P.D. Suppose the key K to be open. Then the 

 current in Ri is the same as that in R z , and its value is 



P.D. 



r 



FIG. 153. Connections 

 for Poggendorf method of 

 comparing a potential differ- 

 ence with an electromotive 

 force. 



/ = 



Rl 



The potential difference between the ends of R\ will consequently 

 be 



+ 



