The unit of the electric resistance, measured with direct current, is the ohm (abbreviated Ω), named after the German physicist and mathematician Georg Simon Ohm (1789-1854). According to ohm’s law, the resistance R is the ratio of the voltage U across a conductor and also the existing I flowing v it:

Hence the follows: 1 Ω = 1 V/A. This definition of the SI ohm is indeed not viable for a realisation.

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Due to the extraordinary reproducibility the the quantum hall resistance, that perfect irreversible stability and world-wide uniformity, the ohm can be realised together a certain fraction of the von-Klitzing constant. Already since 1990, on basis of a referral by the CIPM (Comité worldwide des Poids et Mesures), resistance comparisons and also calibrations world-wide had actually to be described a addressed numerical worth of the von-Klitzing constant, RK-90 = 25812.807 Ω90. The advent of this conventional reference value for the von Klitzing constant had substantial practical advantages in terms of maintainance and also dissemination the the unit ohm. At the same time, however, this additionally meant the the standard unit Ω90 was not compliant v the global System of devices (SI) precious at the time. An SI-realization of the ohm was, e.g., possible with a Thompson-Lampard capacitor (calculable capacitance; because of the intricacy of the equivalent measurement setup, the achievable accuracies were inferior to the reproducibility of quantum hall resistors.

On might 20, 2019, a review of the SI come into force according come which one SI value for the von Klitzing constant RK = h/e2 deserve to be obtained using exactly identified values because that the elementary charge e and also the Planck continuous h. This made it feasible to establish the ohm via the usage of quantum hall resistors within the SI.

At, the resistance unit is realised from the quantum room resistance. For this purpose, ours working team operates a cryostat with a superconducting solenoid. Come guarantee the the hall resistance takes the exactly quantised value, part internationally accepted criteria need to be fulfilled . Firstly, the longitudinal resistance need to be zero since a vanishing longitudinal resistance is a measure up for complete quantisation (otherwise a correction needs to be applied). Furthermore, all call resistances that the quantum Hall device have come be sufficiently small. Prior to every calibration, these criteria need to be showed experimentally. Moreover, the resistance worths calibrated at and also at other national metrology institutes need to be compared from time come time, to guarantee a world-wide energy of the resistance unit ohm.

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for the circulation of the unit, it turned the end to be practicable to calibrate a typical 100Ω resistor with recognized drift behaviour about two times a year, utilizing a cryogenic current comparator. With this 100Ω working resistor, the calibrations because that’s customers are carried out through Working team 2.11. Just in the instance of unique calibrations inquiry a family member uncertainty of 10-9 (or less), the resistor to it is in calibrated is straight measured versus the quantum hall resistance (i.e., there is no the intermediate step with the 100Ω resistor). An instance is a precision measurement at graphene in the structure of a study project.


Ensemble the 1Ω resistors native "The Leeds & Northrup Co." as offered at former times for the conservation of the ohm.

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Head the Working group

Dr. Hansjörg Scherer


Physikalisch-Technische Bundesanstalt Bundesallee 100 38116 Braunschweig