Volt

Volt
Josephson junction array chip developed by the National Bureau of Standards as a standard volt
Unit system	SI derived unit
Unit of	Electric potential, electromotive force
Symbol	V
Named after	Alessandro Volta
In SI base units:	kg·m2·s−3·A−1

The volt (symbol: V) is the derived unit for electric potential, electric potential difference (voltage), and electromotive force.

One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it. It can be expressed in terms of SI base units (m, kg, s, and A) as

It can also be expressed as amperes times ohms (current times resistance, Ohm's law), watts per ampere (power per unit current, Joule's law), or joules per coulomb (energy per unit charge), which is also equivalent to electronvolts per elementary charge:

The "conventional" volt, V₉₀, defined in 1988 by the 18th General Conference on Weights and Measures and in use from 1990, is implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the caesium frequency standard. For the Josephson constant, K_J = 2e/h (where e is the elementary charge and h is the Planck constant), the "conventional" value K_J-90 is used: