Comments on "Influence of voltage contacts on precision measurements of the quantized Hall resistance: an effect of externally injected current"

T.P. Chen and H.A. Chua (ibid., vol. 47, pp. 592-4, 1998) proposed a model to explain the influence of the voltage contact resistance R, on the value of the Hall resistance R/sub H/ in high-precision measurements. Their model is based on the observation that the offset current of the null detector of the cryogenic bridge generates a voltage in the contact resistance that induces a deviation of the Hall resistance from its ideal value. Using their model, they give a quantitative interpretation of a set of high-precision measurements made at the Swiss Federal Office of Metrology. The main argument of the paper is as follows: the offset current of the null detector I/sub offset/ will develop an error voltage V/sub d/=I/sub offset/R/sub c/, where R/sub c/ is the contact resistance. We think this argument is wrong. In addition to the contact resistance, the Hall resistance is present between the terminals of the voltmeter and, therefore, the error voltage is given by V/sub d/=I/sub offset/(R/sub c/+2R/sub H/). In this case, RH is multiplied by two because we are comparing two Hall resistances.