High accuracy determinations of 2e/husing the ac Josephson effect require correspondingly accurate measurements of small dc voltages. In the preliminary stages of a determination of 2e/hwith substantially improved accuracy, two different potentiometric instruments based on two independent principles of operation were designed and built. Both instruments were used in an extended series of 2e/hmeasurements at the University of Pennsylvania and subsequently at the National Bureau of Standards where they have been used to realize the U. S. legal volt as recently redefined via the ac Josephson effect. One of these instruments is based on a modified series‐parallel technique and is described in detail in this paper. After treating the general theory of the low resistance case, the design and construction of a portable self‐checking instrument based on the double series‐parallel exchange method is described. This series‐parallel comparator (SPC), built to compare a Josephson device (10 mV) and a standard cell (1 V), was evaluated and tested in the course of determinations of 2e/hwith one standard deviation (1 &sgr;) uncertainties of about 0.1 ppm. The sources of systematic uncertainty are discussed. The uncertainty attributable to the comparator alone is about 3 parts in 108(1 &sgr;).