# Volt

File:NISTvoltChip.jpg
Josephson junction array chip developed by NIST as a standard volt.

The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force. It is named in honor of the Lombard physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first modern chemical battery.

## Definition

The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m2 · kg · s-3 · A-1, which can be equally represented as one joule of energy per coulomb of charge, J/C.

${\mbox{V}}={\dfrac {\mbox{W}}{\mbox{A}}}={\dfrac {{\mbox{W}}\cdot {\mbox{s}}}{{\mbox{A}}\cdot {\mbox{s}}}}={\dfrac {\mbox{J}}{\mbox{C}}}={\dfrac {{\mbox{N}}\cdot {\mbox{m}}}{{\mbox{A}}\cdot {\mbox{s}}}}={\dfrac {{\mbox{kg}}\cdot {\mbox{m}}^{2}}{{\mbox{A}}\cdot {\mbox{s}}^{3}}}$ ### Josephson junction definition

Since 1990 the volt is maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as

K{J-90} = 0.4835979 GHz/µV.

## Hydraulic analogy

In the hydraulic analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage is likened to water pressure – it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time (volumetric flow rate). The flow rate is determined by the width of the pipe (analogous to electrical resistance) and the pressure difference between the front end of the pipe and the exit (potential difference or voltage). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (amperes x volts = watts).

The relationship between voltage and current (in ohmic devices) is defined by Ohm's Law.

## Common voltages

File:Electronic multi meter.jpg
A multimeter can be used to measure the voltage between two points
File:BateriaR14.jpg
1.5 V C-cell batteries

Nominal voltages of familiar sources:

Note: Where 'RMS' (root mean square) is stated above, the peak voltage is ${\sqrt {2}}$ times greater than the RMS voltage for a sinusoidal signal centered around zero voltage.

## History of the volt

In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force. At that time, the volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.

Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972. 