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

Hence the follows: 1 Ω = 1 V/A. This an interpretation of the SI ohm is without doubt not viable for a realisation.

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Due to the extraordinary reproducibility the the quantum room resistance, the perfect irreversible stability and also world-wide uniformity, the ohm can be realised together a certain fraction of the von-Klitzing constant. Already since 1990, on communication of a recommendation by the CIPM (Comité global des Poids et Mesures), resistance comparisons and also calibrations world-wide had actually to be described a addressed numerical value of the von-Klitzing constant, RK-90 = 25812.807 Ω90. The advent of this conventional referral value because that the von Klitzing continuous had significant practical benefits in terms of maintainance and dissemination that the unit ohm. In ~ the very same time, however, this likewise meant the the typical unit Ω90 was no compliant v the worldwide System of units (SI) valid at that time. One SI-realization the the ohm was, e.g., possible with a Thompson-Lampard capacitor (calculable capacitance; due to the intricacy of the equivalent measurement setup, the achievable accuracies were inferior to the reproducibility that quantum room resistors.

On might 20, 2019, a revision of the SI come into force according to which one SI worth for the von Klitzing continuous RK = h/e2 can be obtained using exactly identified values because that the elementary charge e and the Planck continuous h. This made it feasible to establish the ohm via the use of quantum room resistors in ~ the SI.

At, the resistance unit is establish from the quantum room resistance. Because that this purpose, our working group operates a cryostat v a superconducting solenoid. To guarantee that the room resistance take away the exactly quantised value, some internationally embraced criteria have to be fulfilled . Firstly, the longitudinal resistance have to be zero because a noodles longitudinal resistance is a measure up for complete quantisation (otherwise a correction needs to be applied). Furthermore, all call resistances the the quantum Hall device have to be sufficiently small. Before every calibration, this criteria need to be proved experimentally. Moreover, the resistance values calibrated at and also at other national metrology institutes have to be contrasted from time come time, to guarantee a world-wide energy of the resistance unit ohm.

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because that the circulation of the unit, the turned out to it is in practicable to calibrate a traditional 100Ω resistor with well-known drift behaviour about two time a year, using a cryogenic existing comparator. V this 100Ω functioning resistor, the calibrations for’s client are lugged out by Working group 2.11. Only in the situation of unique calibrations request a loved one uncertainty that 10-9 (or less), the resistor to be calibrated is straight measured versus the quantum hall resistance (i.e., there is no the intermediate action with the 100Ω resistor). An instance is a precision measurement at graphene in the frame of a research study project.


Ensemble that 1Ω resistors from "The Leeds & Northrup Co." as used at former times because that the preservation of the ohm.

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Head that Working group

Dr. Hansjörg Scherer


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