Abstract: An electrometer (20) for measuring an unknown DC potential finds particular utility in applications requiring a high input resistance and minimal current drain during measurement. The electrometer (20) employs a circuit containing a sampling diode (D1) and capacitor (C1). Prior to a measurement being taken, the capacitor (C1) is charged to a common-mode voltage and the diode (D1) is reverse biased to a potential approximating the unknown potential of the voltage source (30) to be measured. A switch (S1) is employed to momentarily cause current flow through the capacitor (C1) and to forward bias the diode (D1). The unknown potential is calculated from potentials associated with the sampling diode (D1) and the capacitor (C1). The calculation involves sampling the potential of the capacitor (C1) at precisely determined time delays.