Techniques for Traceable Measurements of Small Currents

The Italian national standard of direct current is being extended down to the femtoampere range. Two current sources have been developed. The first one, working in the 100 pA to 100 fA range, is based on a low-frequency trapezoidal wave generator, measured with a sampling digital voltmeter at regular time intervals, which charges and discharges a gas-dielectric capacitor. The second, working in the 10-muA to 10-pA range, makes use of a calibrated resistor to generate a known current. Current is traceable to national standards of voltage, resistance or capacitance, and frequency