Internally Damped CCC for Accurate Measurements of Small Electrical Currents

We have developed and tested an internally damped cryogenic current comparator (CCC) with a main 1:20 000 winding ratio for the accurate measurement of small electrical currents. The key features of the CCC are the use of resistive CuNi wire for the windings and brass internal shields for additional damping of the internal resonance of the CCC. Inductance measurements of the CCC overlapping tube at 300 and 77 K, as well as the superconducting quantum interference device (SQUID) noise spectra of the complete CCC setup at 4 K, confirm the effectiveness of the damping of the CCC self-resonance with respect to other designs. At 4 K, the CCC is remarkably stable in measurements with external feedback mode; even mains-operated current sources can be used without unlocking the SQUID null detector of the measurement bridge. First measurements of currents in the range of 100-300 pA using this CCC are performed, resulting in deviations from the expected value in the range of about 20-50 muA/A.