# SI unit: Electricity

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VTT

## 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.

## 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 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 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
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### Ilkka Iisakka

Research Scientist