Cryogenic current comparators are used in electrical metrology to compare ratios of two electrical currents with the highest precision. This is, for example, needed for high precision measurements of resistances, or for the amplification of small currents.
A cryogenic current comparator can be considered an ideal dc transformer. The operating principle is based on the use of the Meißner-Ochsenfeld-Effect in a superconducting screen which surrounds current-carrying coils of different turns, and on the use of a SQUID (Superconducting Quantum Interference Device) used as a null detector for the magnetic field near the screen. Both is based on effects of superconductivity and therefore requires very low temperatures, typically supplied by liquid helium (4 Kelvin). For the operation of cryogenic current comparators in resistance bridges specially designed synchronized dual current sources are needed.
The working group is pursuing technical improvements and innovations in these fields and has achieved significant results.
Licensed by the PTB, corresponding systems are manufactured and distributed by Magnicon GmbH (Hamburg).
Cryogenic current comparator acting as ideal direct current transformer: Coils with different numbers of turns (red, blue) within the superconducting screen (gray) are used for the realization of integer current ratios. The currents I1/2 flowing in the coils cause a screening current to flow in the superconducting screen, which in turn produces an external magnetic field Bext. The screening current and the external magnetic field are, in accordance with the Ampere's law, proportional to I1.N1 - I2.N2. Under conditions of balanced flux, i.e. if for the currents in accordance with the numbers of turns of the coil system I1/I2 = N2/N1 applies, the magnetic field Bext disappears. This is detected very sensitively by the SQUID detector (magnetometer).
Photo of a cryogenic current comparator system developed by the working group. The comparator torus, comprising different coils with in total more than 18 000 turns and a SQUID, is located in the cylindrical screening container (foreground), which for operation is immersed in a liquid helium vessel (background right). For the application in a resistance bridge a special digital dual current source was developed (background left).
Publications of the working group in this field
For further reading:
- J. Gallop, F. Piquemal, „SQUIDs for standards and metrology“, in The SQUID Handbook, J. Clarke and A. I. Braginski, Eds. Weinheim, Germany: Wiley-VCH (2006)
- K. Harvey, „A precise low temperature dc ratio transformer“, Rev. Sci. Instr. 43, 1626 (1972)