Calibration of Current Integrators for Small Beam Currents

The current-time method of calibrating beam current integrators has been extended to give reliable calibrations for small currents when used with a feedback electrometer. The method described has given calibrations with absolute standard errors ranging from 0.2% at 10−10 amp to 0.3% at 10−11 amp. The equipment needed, including a precision 100-megohm resistance, is described, and some experimental difficulties in making calibrations for small currents are discussed.