DC resistance

Instruments calibrated

Standards for electrical resistance

Nominal value [Ω]	Expanded measurement uncertainty [µΩ/Ω] *
0,0001	± 40
0,001	± 3
0,01	± 2
0,1	± 0,6
1 til 10 k	± 0,4
100 k	± 0,7
1 M	± 2
10 M	± 4
100 M	± 8
1 G	± 18
10 G	± 200
100 G	± 2000

* Best measurement uncertainty assumes a high-quality standard and favourable measurement current.

Calibration

A calibration period of one week is calculated for five resistors, provided the calibration time has been agreed/reserved in advance. In addition, the resistors should be allowed to stabilise for a couple of days after transport to the laboratory before measurements begin. Calibrations of air-cooled resistors are carried out at 23 °C, while oil-bath resistors are calibrated at 20 °C or 23 °C.

Hamon step standard with 10 resistors connected in series. Using short-circuit contacts, a parallel connection can be made with 1:100 of the resistance value.

Realisation of the ohm

The unit of electrical resistance, the ohm, is realised using the quantised Hall effect. This effect can be observed in special semiconductor devices in strong magnetic fields at extremely low temperatures. When the strength of the magnetic field is varied, the resistance changes in discrete steps. The value of these steps is given by R = RK /n where R is the electrical resistance, RK is a constant, and n is an integer.

The factor RK is called the Klitzing constant and has the value 25 812.807 Ω. This value is internationally agreed upon and used by all laboratories that realise the quantised Hall effect, ensuring that the method provides the same value for the unit ohm as that obtained from the SI definition. The Klitzing constant is equal to the ratio h/e², where h is Planck’s constant and e is the elementary charge.

On 20 May 2019, the SI units were revised.

All resistors and the quantum Hall resistance therefore increased in value by a factor of 17.793 × 10⁻⁹ when measured with the new SI units.