The unit the the electric resistance, measure up with straight 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 throughout a conductor and also the present I flowing with it:

Hence it follows: 1 Ω = 1 V/A. This an interpretation of the SI ohm is certainly not viable for a realisation.

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Due come the particularly reproducibility of the quantum hall resistance, the perfect irreversible stability and world-wide uniformity, the ohm deserve to be realised together a certain portion of the von-Klitzing constant. Currently since 1990, on basis of a recommendation by the CIPM (Comité international des Poids et Mesures), resistance comparisons and calibrations world-wide had to be described a solved numerical worth of the von-Klitzing constant, RK-90 = 25812.807 Ω90. The advent of this conventional recommendation value for the von Klitzing consistent had significant practical benefits in terms of maintainance and dissemination of the unit ohm. In ~ the exact same time, however, this also meant that the traditional unit Ω90 was not compliant with the global System of devices (SI) precious at that time. An SI-realization that the ohm was, e.g., possible with a Thompson-Lampard capacitor (calculable capacitance; due to the complexity of the equivalent measurement setup, the achievable accuracies to be inferior to the reproducibility the quantum room resistors.

On may 20, 2019, a review of the SI came into force according to which an SI worth for the von Klitzing consistent RK = h/e2 can be acquired using exactly identified values because that the elementary charge e and the Planck consistent h. This make it possible to realize the ohm via the use of quantum hall resistors within the SI.

At, the resistance unit is realised from the quantum room resistance. For this purpose, our working group operates a cryostat through a superconducting solenoid. Come guarantee that the room resistance take away the specifically quantised value, part internationally embraced criteria have to be fulfilled . Firstly, the longitudinal resistance need to be zero since a vanishing longitudinal resistance is a measure for complete quantisation (otherwise a correction needs to be applied). Furthermore, all call resistances the the quantum Hall machine have to be saturated small. Prior to every calibration, these criteria need to be proved experimentally. Moreover, the resistance values calibrated in ~ and at other national metrology institutes need to be contrasted from time to time, to insurance a world-wide uniformity of the resistance unit ohm.

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because that the circulation of the unit, it turned the end to be practicable to calibrate a standard 100Ω resistor with well-known drift behaviour around two times a year, utilizing a cryogenic present comparator. Through this 100Ω functioning resistor, the calibrations for’s customers are lugged out by Working group 2.11. Just in the instance of special calibrations inquiry a loved one uncertainty the 10-9 (or less), the resistor to be calibrated is directly measured versus the quantum hall resistance (i.e., there is no the intermediate action with the 100Ω resistor). An example is a precision measurement at graphene in the structure of a research project.


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

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Head the Working group

Dr. Hansjörg Scherer


Physikalisch-Technische Bundesanstalt Bundesallee 100 38116 Braunschweig