SOME MKRrri.'Y STANDARDS OF RESISTANCE, ETC. 



91 



reduced by 0-0030 per cent. Applying this correction, we have the resistance of the 

 mercury column at 0C. between two planes terminating the bore of the tube, equal 

 to 1 -00006- ohms, the unit of resistance being as previously specified. 



Mcrixiininent of /{t-xistdnce : (b) Potentiometer Method. 



Immediately after the determination by the double bridge, an oljservation by the 

 potentiometer was made. It is convenient to describe this method now, l>efore 

 considering some of the possible errors. 



The diagram (fig. 13) explains the connections. 



76 OHMS 



VVWVWWWVXAAAA/ 



-A/WWWV 



800 OHMS 



Fig. 13. 



P is the mercury standard, P the comparison coil Wolff 1690, as before, X a 

 standard 1-ohm coil of rather high value, and which when employed was evaluated 

 by comparison with P. The additional resistance in this circuit was about 78 ohms, 

 and the value of the current employed averaged 0'025 ampere. 



In the second circuit, Z was a 10-ohm manganin coil, and the current employed 

 averaged 0'0025 ampere. T was a turning head, arranged so that the potential 

 points of either of the three resistances X, F, and P could be quickly connected 

 through the galvanometer to the potential points of Z. The galvanometer resistance 

 was 5 ohms, the deflections being olwerved telescopically, as l>efore, at a distance of 



N 2 



