Cryocurrent comparator
Cryo (engl. Cryogenic current comparator , CCC) used in the electric precision measuring technique to electric currents to be compared and to determine current ratios with the highest accuracy. They exceed the accuracy of other current comparators by several orders of magnitude and are used in electrical metrology, for example, for high-precision comparative measurements of electrical resistances or for amplifying and measuring extremely small currents.
The principle of the cryogenic current comparator goes back to Harvey and is essentially based on the properties of superconductors [Harvey, IK: A precise low temperature DC ratio transformer. Rev. Sci. Instrum. 43 (1972) 1626-39]. Macroscopic quantum effects are used here. which occur in superconductors below the critical temperature of typically a few Kelvin . The term “cryogenic current comparator” is therefore derived from κρυος (Greek for frost , ice ) and comparare (Latin for compare ).
The quantum effects advantageously used in the cryocurrent comparator are on the one hand the ideal diamagnetism of the superconductor, due to the Meissner effect , and on the other hand the properties of a superconducting quantum sensor.
To compare two currents, the currents are passed through two wires that are led through the interior of a superconducting tube. The Meissner effect creates a current on the inside of the tube that is exactly as large as the sum of all currents inside the tube. This shielding current causes a vanishing magnetic field inside the tube. It flows back over the outer surface of the tube and generates a magnetic field outside the tube , which is detected with a highly sensitive magnetometer . The magnetic field detected by the magnetometer is now a measure of the equality of the currents - in particular, it disappears when the two currents to be compared are equal. The important thing is that the strength of the shielding current and the current distribution on the outside of the pipe are independent of the arrangement or position of the wires inside the pipe.
As a zero detector for the magnetic field to SQUID magnetometer (SQUID = S uperconducting Qu antum I nterference D evice) used. They allow the detection of extremely small changes in the magnetic field, which correspond to fractions of the elementary magnetic flux quantum = h / 2 e ≈ 2 · 10 −15 Vs ( h is Planck's constant , e is the elementary charge ). The function of the SQUID is based on macroscopic quantum interference , which occurs in superconducting loops with tunnel contacts.
Resistance bridges based on cryocurrent comparators are used to compare electrical resistances, especially for measurements of the highest precision, as required for reproducing the unit of resistance on the basis of the quantum Hall effect (QHE). Connection measurements of standard resistances in the range from 1 ohm to 10 kOhm to a QHE resistance of 12.9 kOhm are carried out in this way at metrological state institutes such as the Physikalisch-Technische Bundesanstalt (PTB) with a relative measurement uncertainty of only a few 10 −9 .