The DC Voltage Calibrator is a portable, low-impedance source of highly accurate and stable dc voltage. The instrument supplies voltages from 0.00000 to ±1111.110 volts, which are selected by a range multiplier dial and six decade dials. The range dial has nominal full-scale 10-, 100-, and 1000-volt ranges, and indicates the decimal-point placement, in the readout of the decade dials, according to the range selected. The range dial also has a zero-volt-output position. The decade dials control detented, bidirectional, make-before-break, rotary switches. Output voltages higher than 1000 volts are obtained by the use of the overrange positions marked X (marked 10 on the extreme left-hand dial) on the decade dials. The output current capability of the instrument is 25 milliamperes nominal at all output voltage settings. The instrument is protected by an overcurrent circuit which reduces the output voltage to zero and flashes the front panel indicator if the output current exceeds the nominal 25 milliamperes. The protective circuit cycles on and off so long as the overcurrent condition exists and then resets automatically when the condition is removed. The output of the instrument is isolated from the ac-power line and the outer case so that the output can be applied to isolated or grounded loads. The metal terminal on the front panel may be used to ground either side of the output.