This article is part of a series detailing the SI units and our calibration services. This article centers around electricity.

## Unit of electric current: ampere (A)

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge *e* to be 1.602 176 634 ×10^{−19} when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆*ν*_{Cs}.

## Unit of electric potential: volt (V)

The volt is the potential difference between two points of a conducting wire carrying a constant current of 1 ampere, when the power dissipated between these points is equal to 1 watt.

## Unit of electric resistance: ohm (Ω)

The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.

## Practical realization of the ampere, volt and ohm

### In practice, the ampere A can be realised:

- by using Ohm’s law, the unit relation A = V/Ω, and using practical realizations of the SI derived units the volt V and the ohm Ω, based on the Josephson and quantum Hall effects, respectively
- by using a single electron transport (SET) or similar device, the unit relation A = C/s, the value of
*e*given in the definition of the ampere and a practical realization of the SI base unit the second s; - by using the relation
*I*=*C*·d*U*/d*t*, the unit relation A = F·V/s, and practical realizations of the SI derived units the volt V and the farad F and of the SI base unit second s.

**In practice, the volt can be realised: **

- using the Josephson effect and the following value of the Josephson constant:
*K*_{J}= 483 597.848 416 984 GHz V^{–1}. This value has been calculated to 15 significant digits.

### In practice, the volt can be realised:

- using the Josephson effect and the following value of the Josephson constant:
*K*_{J}= 483 597.848 416 984 GHz V^{–1}. This value has been calculated to 15 significant digits.

### In practice, the ohm can be realised:

- by using the quantum Hall effect in a manner consistent with the CCEM Guidelines and the following value of the von Klitzing constant
*R*_{K}= 25 812.807 459 3045 Ω. This value has been calculated to 15 significant digits. - by comparing an unknown resistance to the impedance of a known capacitance using.

## Unit of capacitance: farad (F)

The farad is the capacitance of a capacitor between the plates of which there appears a potential difference of 1 volt when it is charged by a quantity of electricity of 1 coulomb.

## Unit of electric inductance: henry (H)

The henry is the inductance of a closed circuit in which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at the rate of 1 ampere per second.

## Electrical metrology calibration services

- Alternating voltage and current
- capacitance, resistance, direct voltage and current,
- RF- and microwave ranges, power meters, measuring devices of electricity (multimeters, calibrators, etc.) and high voltage quantities
- More information in our calibration service brochure