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 ∆νCs.
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.
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.
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·dU/dt, 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: KJ = 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 RK = 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 determined, for example, by means of a calculable capacitor.
Electrical metrology calibration services
- Direct and alternating voltage and current,
- resistance, capacitance, inductance,
- radio frequencies and microwaves,
- power meters,
- high voltage quantities,
- electrical multimeters, calibrators, etc.
- More information in our calibration service brochure