SI units in Finland, electricity

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

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Unit of electric current: ampere (A)

What is the unit of electric current? 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.

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