THE POTENTIOMETER. 23 



necessary that the different amalgams should be maintained at exactly iden- 

 tical temperatures ; hence the care taken about this point. 



The centigrade temperature was read from a thermometer graduated to 

 tenths of a degree and permitting the estimation of hundredths, which was 

 compared from time to time with a Reichsanstalt thermometer. The zero 

 point of the latter was determined in ice and distilled water. Small correc- 

 tions to the bore had been determined by comparison with a very accurate 

 Baudin thermometer. 



THE POTENTIOMETER AND ITS CALIBRATION. 



The potentiometer shown in figure 5 was designed to give direct readings 

 accurate to a few millionths of a volt. A fall of potential of 1 volt in the 

 10,000 ohms between B and P corresponds to 0.00000 1 volt for each 0.01 

 ohm ; therefore, to attain the required degree of precision, all resistances to 

 be included between the poles of unknown cells must be known within a few 

 hundredths of an ohm. Since not more than one-twentieth of the entire fall 

 of potential was ever neutralized in this manner, the other parts of the 

 system need be standardized only one-twentieth as carefully. As a matter 

 of fact the box A, containing nine 100-ohm coils and ten 10-ohm coils, was 

 calibrated to 0.01 ohm, and the external resistances to 0.1 ohm. For the 

 purpose a method of substitution, very similar to one recommended by 

 Ostwald, 43 was adopted. In each comparison all four arms of the comparing 

 bridge were of the same order of magnitude, so that the most sensitive and 

 reliable adjustment was secured. All connections whose resistance could 

 influence the results were made with great care. Extra heavy brass con- 

 nectors were clamped upon the polished pegs of box A with the help of 

 pliers ; heavy copper wires were used as leads, and their ends were either 

 soldered or amalgamated and dipped in the mercury cups of the rocker switch. 

 The plugs used in comparing the bridge box were polished and driven home 

 so well that the total resistance of the bars and connections measured only 

 0.006 ohm a negligible amount. Heavy wires were used in the rocker 

 switch, and all their joints were soldered. 



The table of corrections was constructed as if the resistances had been 

 weights, 44 the unit of reference being the value of the whole potentiometer 

 box as 1,000 nominal ohms. The accuracy of the table was established in a 

 wholly satisfactory manner by the direct comparison of several nominally 

 equal combinations ; in every case the observed results coincided with those 

 predicted from the table. 



43 Ostwald-Luther, Hand- und Hulfsbuch, p. 355. 

 "Richards, Journ. Am. Chem. Soc, 22, 144 (1900). 



