Two current sources which give a constant current over a wide range of applied voltage have been built for use in calibrating electrometers having input resistors in the 10+10to 10+12&OHgr; range. The sources consist of sealed parallel plate ionization chambers having one electrode coated with241Am. Calibrations using the rate of charge method spaced over a period of 1 year define the respective current output of the sources as (6.943±0.003)×10−11and (4.263±0.004)×10−12A.