volt

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,

$a fraction. The numerator is meters squared times kilograms. The denominator is seconds cubed times amperes.$

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

Some everyday voltages

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	240 V (alternating current)
transmission lines	69kV, 138 kV, 230 kV, 345 kV, 500 kV, 700 kV, 1,000 kV

History

The cgs unit of electromotive force (e.m.f.) was based on an idea of F. E. Neumann in 1825. 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, 1884) as 10⁸ 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 10⁸ 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 ampere equals 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, K₁, 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 extending the number of decimal places to which voltage could be measured.