National Physical Laboratory

Cryogenic Current Comparator

The cryogenic current comparator (CCC) is a device, which enables the accurate measurement of the ratio of two direct currents. The CCC relies on superconductivity to achieve this and therefore operates at low temperatures, typically that of liquid helium (4.2 K). The principle of operation utilises the exclusion of magnetic flux from the bulk of a superconductor (Meissner effect) and Ampere's law, which states that the path integral of magnetic flux along a closed loop equals the current linking this loop (i.e. the surface integral of current density over the surface enclosed by the loop).

The general shape of a CCC is that of a long, superconducting tube, threaded by two conductors, which carry the currents to be compared.Due to the Meissner effect and Ampere's law, a current flows along the inner surface and back along the outer surface of the tube equal and opposite to the net sum of the currents in the conductors threading the tube. This is the case because well inside the wall of the tube the magnetic flux density is zero (Meissner effect), and therefore the sum of all the currents threading any loop, which lies well inside the wall of the tube and encircles its centre, must also be zero (Ampere's law). Any deviation of the net sum of the currents in the current-carrying conductors from zero results in a surface current along the tube. The resulting surface current on the outside of the tube can be measured using a detection coil, typically connected to a SQUID (Superconducting Quantum Interference Device), which enables the measurements of very small magnetic flux at low temperatures.

Cryogenic Current Comparitor
Operating principle of cryogenic current comparator. Any imbalance in the currents carried by the two conductors I1 - I2 causes an equal and opposite screening current IS along the surface of the CCC shield.In reality, this screening current is distributed around the tube and the diagram only shows a representative path. On the outside, far away from the ends, the surface current is distributed symmetrically and can be detected by a detection coil (not shown).

In practice the shape of the CCC is modified in order to accommodate windings, which thread the CCC several times, enabling different integer ratios of current to be established. Also, a more compact design is achieved, and any effects due to the finite length of the tube can be eliminated.

Accurate current ratios can be established by controlling one of the currents based on the signal measured by the SQUID and adjusting this current to null the SQUID signal.

The ability to establish accurate current ratios is used in cryogenic current comparator resistance bridges where the two currents are passed through two resistors and the voltage difference is measured, enabling highly accurate measurements of the resistance ratios. At NPL, dc resistance calibrations are carried out on a routine basis using cryogenic current comparator bridges, covering the resistance range from 100 μΩ to 1 GΩ.

References

  • K. Grohmann, H. D. Hahlbohm, H. Lübbig, H. Ramin, 'Ironless Cryogenic Current Comparator for AC and DC Applications', IEEE Trans. Instrum. Meas. Vol. 23, No.4, p. 261-263, 1974.
  • J.M. Williams, A. Hartland, 'An Automated Cryogenic Current Comparator Resistance Ratio Bridge', IEEE Trans. Instrum. Meas. Vol. 40., No. 2, p.267-270, 1991.
Last Updated: 12 Dec 2012
Created: 8 Jun 2007

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