An Integrated System for the Precise Calibration of Four-Terminal Standard Resistors

A description is given of the system currently in use at the National Bureau of Standards for measurements of 1-?? standard resistors of the Thomas type. A tenfold improvement in accuracy over the former method has been realized. Resistors of this type are now reported to eight decimal places with a total uncertainty of 0.08 ppm. The latter figure includes a three-standard-deviation limit for random errors plus an estimate of systematic errors [1]. The new system is not only more rapid in operation than the old one (fewer man hours required), but has consistently produced data of superior resolution while operating with a lower test power in resistors. A dc current-linkage balance is used to determine the standard-unknown differences while both are connected in series and totally immersed in a specially designed circular oil bath, which remains completely covered during all tests. Bath temperature is maintained constantly at 25 000?? (??0.003??C) in a unique manner.