# volt

Physical unit
Unit name volt
Unit symbol ${\ displaystyle \ mathrm {V}}$
Physical quantity (s) Electric voltage
Formula symbol ${\ displaystyle U}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {2} \; T ^ {- 3} \; I ^ {- 1}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, V = 1 \; {\ frac {W} {A}} = 1 \; {\ frac {J} {C}}}}$ ${\ displaystyle \ mathrm {{} = 1 \; {\ frac {kg \, m ^ {2}} {A \, s ^ ​​{3}}} = 1 \; {\ frac {N \, m} { A \, s}}}}$
Named after Alessandro Volta
Derived from Kilogram , meter , ampere , second

The volt is the unit of measurement used in the International System of Units (SI) for electrical voltage . It was named in 1897 after the Italian physicist Alessandro Volta . The capital letter "V" is used as the unit symbol .

The volt is a coherent derived SI unit , that is, it is linked to the base units by the numerical factor one.

## Definition and representation

### Definition as an SI unit

In the SI system of units, the volt is defined as the electrical potential difference or electrical voltage that is present when a current of one ampere (1 A) produces one watt (1 W) of power.

${\ displaystyle \ mathrm {1 \, V = 1 \, {\ frac {W} {A}}}}$

The watt is connected to the energy unit Joule (J) and the unit of force Newton  (N) via:

${\ displaystyle \ mathrm {1 \, J = 1 \, N \, m = 1 \, W \, s} \,}$,

and thus:

${\ displaystyle \ mathrm {1 \, V = 1 \, {\ frac {N \ cdot m} {A \ cdot s}} = 1 \, {\ frac {kg \ cdot m ^ {2} / s ^ { 2}} {A \ cdot s}} = 1 \, {\ frac {kg \ cdot m ^ {2}} {A \ cdot s ^ {3}}}} \ ,.}$

### Reproducible representation

The above definition is difficult to use as an accurate reference in practice. Since 1990 one volt has been realized as a voltage standard using the Josephson effect . The voltage is linked to a frequency measurement via the Josephson constant . This constant is

${\ displaystyle K _ {\ mathrm {J}} = {\ frac {2e} {h}} = 4 {,} 835 \, 978 \, 484 \ ldots \ cdot 10 ^ {14} \, \ mathrm {\ frac {Hz} {V}} \ ,.}$

It is known exactly because the elementary charge and Planck's quantum of action are assigned exact values ​​(see also definition of SI units ). The representation of the volt can thus be traced back to a very precise frequency measurement (time measurement) by means of reference measurements. ${\ displaystyle e}$ ${\ displaystyle h}$

### Historical definition and naming

In 1861 the two English electrical engineers Josiah Latimer Clark and Charles Tilston Bright suggested naming the unit of electrical voltage with Ohma (after the German physicist Georg Simon Ohm ) and the unit of electrical resistance with volt . In 1881 the International Electricity Congress finally established the volt as the unit for electrical voltage and the ohm as the unit for electrical resistance.

In the 19th century, the units amperes (A) and ohms (Ω = V / A) were defined by standards: the ampere via the amount of silver per period that is deposited during electrolysis, and the ohm via a metallic resistance standard. This also implicitly determined the volt. These definitions were initially country-specific (in Germany 1888) and were standardized internationally in 1908. In 1910 the volt was defined via the Weston normal element , and the ampere was also redefined. The “international” units V int and Ω int defined in this way were chosen from the start in such a way that they should match the “absolute” units V abs and Ω abs as exactly as possible , which have the relationships 1 V abs = 1 A · Ω abs and 1 V abs · A · s = 1 N · m. The deviation was 3.4 · 10 −4 and 4.9 · 10 −4, respectively .

By resolution of the CIPM in 1946, ratified in 1948 by the 9th General Conference on Weights and Measures , the “international” volt was abolished as a separately defined unit; the voltage standard only served to represent the volt.

In 1988 the CIPM decided on a new voltage standard with effect from 1990, based on the Josephson effect . The value K J-90  = 483 597.9 GHz / V was established for the Josephson constant . With the revision of the SI in 2019 , the Josephson constant received its current, exact value.

## Common decimal multiples

The unit volt is used with various prefixes for units of measurement ( SI prefixes ), for example:

Prefix notation Decimal
1 µV (microvolt) 0.000 001 V
1 mV (millivolts) 0.001 V
1 V (volt) 1 V
1 kV (kilovolt) 1,000 B.C.
1 MV (megavolt) 1,000,000 BC