The cryogenic current comparator (CCC) is used in the electrical precision measurements to compare
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
s with the highest accuracy. This device exceeds the accuracy of other current comparators around several orders of magnitude. It is used in electrical
metrology
Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...
for exact comparative measurements of
electric resistances or 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
Critical or Critically may refer to:
*Critical, or critical but stable, medical states
**Critical, or intensive care medicine
*Critical juncture, a discontinuous change studied in the social sciences.
*Critical Software, a company specializing in ...
of typically a few
kelvins. 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
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
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
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
, 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 is independent of the position and the path of the wires inside the tube.
Typical for a CCC is the use of a
SQUID magnetometer
True squid are molluscs with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not st ...
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''/2''e'' ≈ (''h'' is the
Planck constant and ''e'' the
elementary charge
The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
). 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 from 1
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
up to 10 kΩ to a QHE resistor of 12.9 kΩ are performed at several national institutes of metrology as, for instance, th
National Institute of Standards and Technology (NIST, USA)or th
Physikalisch-Technische Bundesanstalt (PTB, D) Here, electrical resistance comparisons using CCCs are accomplished with relative
measurement uncertainties of only about 10
−9.
References
{{reflist
External links
* J.M. William
Cryogenic current comparators and their application to electrical metrology 2011,
doi:10.1049/iet-smt.2010.0170
Electronic test equipment