## Definition

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.^{
[2]} 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

- ${\text{V}}={\frac {\text{potential energy}}{\text{charge}}}={\frac .$

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 electron-volts per
elementary charge:

- ${\text{V}}={\text{A}}\cdot \Omega ={\frac {\text{W}}{\text{A}}}={\frac {\text{J}}{\text{C}}}={\frac {\text{eV}}{e}}.$

### Josephson junction definition

The "
conventional" volt, V_{90}, 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} = 2*e*/*h* (where *e* is the
elementary charge and *h* is the
Planck constant), the "conventional" value *K*_{J-90} is used:

- $K_{\text{J-90}}=0.4835979\,{\frac {\text{GHz}}{\mu {\text{V}}}}.$

This standard is typically realized using a series-connected array of several thousand or tens of thousands of
junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design).^{
[3]} Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation.^{
[4]}