Two mains-fed instruments are described, each of which gives easy and precise direct-current measurement of resistance. The range of resistance covered extends in the one instrument (the megohmeter) from 10 000 ohms to 1 200 000 megohms, and in the other (the micromhometer) from 260 ohms to 330 000 megohms. In the megohmeter the value is read on a potentiometer dial which is uniformly divided in units of resistance, and in the micromhometer the value is shown directly by a galvanometer whose scale is uniformly divided in units of conductance. In neither instrument is the scale accuracy affected by changes of valves.The method, used in the micromhometer, of constraining a measuring network to conform to a pre-existing scale, and the procedure adopted for the progressive self-calibration of its many ranges, may be found suitable for other instruments.