New capability for generating and measuring small DC currents at NPL

This paper describes two systems recently developed at the National Physical Laboratory (NPL), Teddington, U.K., for the generation and measurement of dc currents in the range 100 fA to 1 nA. Both systems are based on the capacitor charging technique. One system generates known currents, and constitutes a new reference at NPL. The other was designed to meet the traceability requirements of the Ionizing Radiation sections of NPL and combines commercially available instrumentation and standards to produce a reliable stand-alone system for measuring small currents.