For the meaning of any metric prefix, go here.

The unit of electric potential, potential difference, and electromotive force
in
SI. Symbol, V. If you think of your house wiring as
plumbing, volts measure the water pressure. One volt is the potential difference
between two points on a conductor when the current flowing is 1
ampere and the power dissipated between the points is one
watt (definition adopted by the CIPM
in 1946, Resolution 2). The volt is a
derived
unit, ^{watt}⁄_{ampere}, or in terms of base units only,

It is named for the Italian physicist Count Alessandro Giuseppe Anastasio Volta (1745 – 1827).

flashlight battery | 1.5 volts |

automobile battery | 12 volts (formerly 6 V, and to be changed to 42 V) |

household current (U.S.) | 117 volts (alternating current) |

household current (non U.S.) | see electric power |

distribution lines (U.S.) | 240 V (alternating current) |

transmission lines | 69kV, 138 kV, 230 kV, 345 kV, 500 kV, 700 kV, 1,000 kV |

The cgs unit of electromotive force (e.m.f.) was
based on an idea of F. E. Neumann in 1845. One
cgs unit of e.m.f. was produced in any electric circuit cutting one magnetic
line of force per second. But the electromotive force people were accustomed to
working with was that produced by batteries, which was millions of the cgs unit.
To meet the need for a unit large enough for practical use, the volt was defined
at the First International Conference of Electricians (Paris,
1881) as 10^{8}
cgs units of e.m.f..

However, establishing the volt by using Neumann's equations was far beyond the capabilities of most laboratories. They got their standard voltage from special batteries (properly speaking, cells). Cells can be made in a variety of ways, from different materials, and each type has a characteristic voltage. A new carbon-zinc flashlight battery, for example, produces 1.5 volts no matter what size it is. Unlike flashlight batteries, some types of cells are able to maintain an unvarying voltage for years. No power is drawn from such “standard cells”; they are simply a voltage reference.

The Fourth Congress (Chicago, 1893) met the need
for a volt defined by a standard cell by defining the international volt (symbol
V_{int}). A Clark cell (a kind of cell that uses mercury and zinc)
at a temperature of 15° centigrade would by definition have an e.m.f. of 1.434
international volts. (See ohm and
ampere for definitions of their international versions.) The value of 1.434
was chosen to make the international volt equal to 10^{8}
cgs units of e.m.f. within the limits of experimental error of the day.

Later work by national standards laboratories with the international ohm, volt and ampere soon showed that these definitions were inconsistent with the relationship amperes equal volts divided by ohms. To scotch this problem, the International Conference on Electrical Units and Standards (London, 1908) dropped the “reproducible” definition of the international volt, deriving it instead from the international ampere and international ohm: 1 international volt is the potential difference between the ends of a conductor having a resistance of 1 international ohm, when the steady current through it is 1 international ampere.

In 1948 the CGPM abandoned international units in favor of absolute ones. At that time, the international volt was determined to be 1.00034 volts. (If you’re counting, so far we have been through three different definitions of the volt, all leading to more or less identical values in everyday life, and still haven’t reached the current one.)

One of the considerations in the reforms that lead to the replacement of the cgs system of units for scientific work was the need to have an electromagnetic base unit. The ohm was considered for this role, but eventually the ampere was chosen, leading to the the meter-kilogram-second-ampere system, and subsequently to SI. With the ampere a base unit, the volt was defined in terms of the ampere and power, in this case watts, which leads to today's definition, given above. See history of the ampere.

Technical progress led to new, far more precise options for laboratory
standards for the volt. In 1988, on the recommendation
of the Consultative Committee on Electric Units, the CIPM adopted as a
conventional value “483597.7 GHz/V for the Josephson constant, K_{1},
that is to say, for the quotient of frequency divided by the potential
difference corresponding to the n=1 step in the Josephson effect.” (CIPM
Recommendation 1, CI-1988)

The value of the volt established in this manner was to be used by all laboratories after 1 January 1990, regardless of whether they were using this particular type of standard. In 1990, a comparison of the voltage standards of the various national laboratories showed 5 volts in use: US and Taiwan 9 ppm smaller; French 6.5 ppm smaller; USSR 3.3 ppm smaller; other countries 7.8 ppm smaller.

The CIPM was at pains to point out that Recommendation 1 did not amount to a redefinition of the volt. Strictly speaking that is true, but it certainly extended the number of decimal places to which voltage could be measured.

1

Dr. Esselbach's next conclusion is also of great practical value. He points
out that the electro-magnetic unit of electromotive force, also multiplied by
10^{10}, differs extremely little from that of the common Daniell's cell, and that,
without doubt, by proper care such a cell could be constructed as would form
a practical unit of electromotive force. This suggestion has the approval of
the Committee.

First Report of the Committee- Cambridge, October 3, 1862.

Fleeming Jenkin, editor.

Reports of the Committee on Electrical Standards
Appointed by the British Asociation for the Advancement of Science.

London: E & F. N. Spon, 1873.

Page 9

2

In favour of Weber's units it is urged-

1st. That their use
will ensure the adoption of a complete system of corresponding

standards for
electrical currents, quantities, and tension or difference of potential.

2nd. That their use is essential in the dynamic treatment of any problem
connected with electricity; for instance, in determining the heat generated,
the force exerted, the work done, and the chemical action required or produced
under any given circumstances.

3rd. That their use would be a simple
extension of the system already

universally adopted in magnetic measurements.

4th. That the unit is independent of the physical properties of any material.

Against the system it is urged that the unit cannot be determined with
sufficient accuracy, and that even its approximate reproduction, where copies
cannot be obtained, is difficult and expensive.

Page 33, 1st report

The Committee, after mature consideration, are of opinion that the system of so-called absolute
electrical units, based on purely mechanical measurements, is not only the best
system yet proposed, but is the only one consistent with our present knowledge
both of the relations existing between the various electrical phenomena and of
the connexion between these and the fundamental measurements of time, space, and
mass. The only hesitation felt by the Committee was caused by doubts as to the
degree of accuracy with which this admirable system could be or had been reduced
to practice.

Page 40, 2nd Report

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