Cryogenic current comparator
Encyclopedia
The cryogenic current comparator (CCC) is used in the electrical precision measurements to compare electric current
s and/or to determine electric current ratios with highest accuracy. This device exceeds the accuracy of other current comparators around several orders of magnitude and is used in electrical metrology
for highly precise comparative measurements of electric resistances
or for the amplification and measurement of extremely small electric currents.
The CCC principle goes back on Harvey and is based substantially on the properties of superconductors
. CCCs make use of macroscopic quantum effects that occur in superconducting materials or circuits underneath their critical temperature of typically a few kelvin
s. The term “Cryogenic Current Comparator” stems from κρυος (Gr.
frost, ice) and comparare (Lat. compare). The two quantum effects used in a CCC are the ideal diamagnetism
of the superconductor, caused by the Meissner effect
, and the macroscopic quantum interference of currents in a superconducting quantum sensor.
For the comparison of two currents these are fed through two wires which are led through a superconducting tube. The Meissner effect induces a screening current on the inner surface of the tube, flowing opposite to and being exactly as large as the sum of the currents inside the tube. Thus, this shielding current exactly cancels the magnetic field
inside the tube produced by the currents in the wires. The screening current flows back across the outer surface of the tube, giving rise to a magnetic field in the room outside of the tube. This field is detected by a highly sensitive magnetometer
, acting as a null detector. The signal of this null detector thus is a measure for the equality of the currents; in particular it is zero if the two currents are of exactly equal magnitude. The important and crucial point characterizing the CCC is the fact that the magnitude of the screening current and its distribution on the surface of the superconducting screen are independent of the position and the path of the wires inside the tube.
Typical for a CCC is the use of a SQUID magnetometer
as null detector for the magnetic field (SQUID = Superconducting Quantum Interference Device). These are capable of detecting extremely small changes of the magnetic field corresponding to fractions of the magnetic flux quantum
= h/2e ≈ 2×10−15 V·s (h is Planck's constant and e the elementary charge
). The function principle of a SQUID is based on macroscopic quantum interferences of electric currents, arising in superconducting circuits (loops) with tunnel junctions.
Resistance bridges based on CCCs are used for the comparison of electrical resistances, in particular if highest-precision measurements are required, as there is the traceability
of the resistance unit to the quantum Hall effect (QHE). In this way, measurements connecting standard resistors ranging within 1 ohm
up to 10 kΩ to a QHE resistor of 12.9 kΩ are performed at several national institutes of metrology as, for instance, the National Institute of Standards and Technology (NIST, USA) or the Physikalisch-Technische Bundesanstalt (PTB, D). Here, electrical resistance comparisons using CCCs are accomplished with relative measurement uncertainties
of only about 10−9.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
s and/or to determine electric current ratios with highest accuracy. This device exceeds the accuracy of other current comparators around several orders of magnitude and is used in electrical metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
for highly precise comparative measurements of electric resistances
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
or for the amplification and measurement of extremely small electric currents.
The CCC principle goes back on Harvey and is based substantially on the properties of superconductors
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
. CCCs make use of macroscopic quantum effects that occur in superconducting materials or circuits underneath their critical temperature of typically a few kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s. The term “Cryogenic Current Comparator” stems from κρυος (Gr.
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
frost, ice) and comparare (Lat. compare). The two quantum effects used in a CCC are the ideal diamagnetism
Diamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
of the superconductor, caused by the Meissner effect
Meissner effect
The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin...
, and the macroscopic quantum interference of currents in a superconducting quantum sensor.
For the comparison of two currents these are fed through two wires which are led through a superconducting tube. The Meissner effect induces a screening current on the inner surface of the tube, flowing opposite to and being exactly as large as the sum of the currents inside the tube. Thus, this shielding current exactly cancels the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
inside the tube produced by the currents in the wires. The screening current flows back across the outer surface of the tube, giving rise to a magnetic field in the room outside of the tube. This field is detected by a highly sensitive magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
, acting as a null detector. The signal of this null detector thus is a measure for the equality of the currents; in particular it is zero if the two currents are of exactly equal magnitude. The important and crucial point characterizing the CCC is the fact that the magnitude of the screening current and its distribution on the surface of the superconducting screen are independent of the position and the path of the wires inside the tube.
Typical for a CCC is the use of a SQUID magnetometer
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
as null detector for the magnetic field (SQUID = Superconducting Quantum Interference Device). These are capable of detecting extremely small changes of the magnetic field corresponding to fractions of the magnetic flux quantum
Magnetic flux quantum
The magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor. The phenomenon of flux quantization was discovered B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Nabauer, in 1961...
= h/2e ≈ 2×10−15 V·s (h is Planck's constant and e the elementary charge
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
). The function principle of a SQUID is based on macroscopic quantum interferences of electric currents, arising in superconducting circuits (loops) with tunnel junctions.
Resistance bridges based on CCCs are used for the comparison of electrical resistances, in particular if highest-precision measurements are required, as there is the traceability
Traceability
Traceability refers to the completeness of the information about every step in a process chain.The formal definition: Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that is verifiable....
of the resistance unit to the quantum Hall effect (QHE). In this way, measurements connecting standard resistors ranging within 1 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
up to 10 kΩ to a QHE resistor of 12.9 kΩ are performed at several national institutes of metrology as, for instance, the National Institute of Standards and Technology (NIST, USA) or the Physikalisch-Technische Bundesanstalt (PTB, D). Here, electrical resistance comparisons using CCCs are accomplished with relative measurement uncertainties
Measurement uncertainty
In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured...
of only about 10−9.