Electrical resistance is an area of expertise at the National Measurement Laboratory for Electrical Quantities. At our laboratory the resistance unit is realised with the highest possible accuracy. In addition we perform calibrations, measurements, evaluations, and conduct research related to electrical resistance.
The resistance unit, ohm (Ω), is realized by utilizing the Quantum Hall Effect, which is an example of a macroscopic quantum physical phenomenon. The realisation is based on the fact that the resistance in a so-called quantum Hall bar is unambiguously determined by two of the fundamental natural constants which the new SI system is built on. Before 20 May, 2019, the SI system was based on seven base units from which all other units could be derived. The electrical base unit was the unit for electrical current, the ampere (A), and hence the ohm was a derived unit. However, on May 20, 2019, the SI system was revised and instead of seven basic units, the SI system is today based on seven natural constants from which all SI units can be derived. During the revision, the natural constants in question have also been given exact values. By utilizing the Quantum Hall effect, the resistance unit can be derived from Planck's constant h and the elementary charge e. In practice, the ohm has been realized in this way for several decades, but the revision ensures that the same realisation now takes place in accordance with the SI system's definitions. The traceability follows from a numerical value for the ratio between Planck's constant and the elemental charge squared, the so-called von Klitzing constant RK.
After the realisation, the realised resistance unit is transferred to groups of 1 Ω - 10 kΩ primary and secondary standards which maintain the resistance unit. Between realisations these standards are regularly compared to each other to ensure that the resistance unit is maintained with satisfactory accuracy.
Our quantum Hall resistance standards are fabricated in gallium arsenide (GaAs) or in graphene and are of high international standard. Together with Chalmers University of Technology, and through several international collaborations, we participate in research projects aimed at developing tomorrow's graphene-based resistance standards. To ensure traceability, we participate in international comparisons. The measurement uncertainty for the realisation is of the order of 0.013 μΩ / Ω.
Contact us if you have any questions about our work on electrical resistance. We offer calibration of resistance standards and calibrators and can also assist in the development of new calibration methods. More information about our calibration can be found on our service pages.