Cryogenic current comparator
Cryo (English cryogenic current comparator, CCC ) are used in the electrical precision measuring technique to compare electrical currents or to determine current conditions with the highest accuracy. They exceed the accuracy of other current comparators several orders of magnitude and are used in electrical metrology, for example, high-precision comparison measurements of electrical resistors or for amplification and measurement of extremely small currents.
The principle of Kryostromkomparators 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 ]. While macroscopic quantum effect can be utilized. occurring in superconductors below the transition temperature of typically a few Kelvin. The term " cryogenic current comparator " therefore derives from κρυος ( gr frost, ice) and comparare (Latin: Compare ).
In the Kryostromkomparator advantageously exploited quantum effects are on the one hand the perfect diamagnetism of the superconductor due to the Meissner effect, and on the other hand the properties of a superconducting quantum sensor.
For comparing two current values passing the currents through two wires, which are passed through the inside of a superconducting tube. By the Meissner effect, a current is formed on the inside of the tube, which is exactly as large as the sum of the currents in the interior of the tube. This shielding current causes a vanishing magnetic field in the interior of the tube. It flows back over the outer surface of the tube and outside the tube generates a magnetic field which is detected using a highly sensitive magnetometer. The detected magnetic field by the magnetometer is now a measure of the identity of the flows - in particular, it will disappear when the two are equal to the comparative current. Important is the fact that the strength of the Abschirmstromes and the current distribution on the pipe outer surface are independent of the arrangement or position of the wire inside the tube.
The zero detector for the magnetic field SQUID magnetometer (SQUID = Superconducting Quantum Interference Device ) can be used. They allow for the detection of extremely small changes in magnetic field, the fractions of the elementary magnetic flux quantum = h/2e ≈ 2:10 -15 Vs correspond to (h is the Planck constant, e is the elementary charge ). The function of the SQUID based on macroscopic quantum interference occurring in the superconducting tunnel junctions with loops.
Resistance bridges based on cryo be used for the comparison of electric resistances, in particular for high-precision measurements, such as for reproduction of the resistor unit on the basis of the quantum Hall effect ( QHE ) are required. Port measurements of standard resistors in the range 1 ohm to 10 kohm to a QHE resistance of 12.9 ohms in this way at national metrology institutes such as the Physikalisch-Technische Bundesanstalt ( PTB) carried out with a relative uncertainty of only a few 10-9.