SI unit of electrical potential difference; symbol V; named after Alessandro Volta; if the power dissipated between two points along a wire carrying a current of 1 amp is 1 watt, then the potential difference between the two points equals 1 volt. In 1990 the Josephson constant, KJ-90, became the international standard of voltage measurement.
The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force .
Definition
The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.
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. The equation that brings all three components together is: volts × amperes = watts
Common voltages
Nominal voltages of familiar sources:
Nerve cell action potential: around 30 millivolts Single-cell, rechargeable alkaline battery: 1.25 volt Mercury battery 1.355 volts Single-cell, non-rechargeable battery (e.g. AAA, AA, C and D cells): 1.5 volts Lithium polymer rechargeable battery: 3.75 volts Transistor-transistor logic (TTL) power supply: 5 volts PP3 battery: 9 volts Automobile electrical system: 12 volts (nominal) Household mains electricity: 230 volts RMS in Europe, Australia, Asia and Africa, 120 volts RMS in North America, 100 V RMS in Japan Rapid transit third rail: 600 to 700 volts High voltage electric power transmission lines: 110 kilovolts RMS and up (1150 kV is the record as of 2005) Lightning: Varies greatly, often around 100 megavolts.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. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force. 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.
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