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Josephson junction array chip developed by NIST as a standard volt.

The volt (symbol: V) is the SI derived unit of electromotive force, commonly called " voltage". It is also the unit for the related but slightly different quantity electric potential in a point (voltage as related to a reference ground) and electric potential difference (also called "electrostatic potential difference"). It is named in honour of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery.


The volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor. It can be written in terms of SI base units as: m2 · kg · s−3 · A−1. It is also equal to one joule of energy per coulomb of charge, J/C.

\mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} = \sqrt{\mbox{W} \cdot \Omega} = \dfrac{\mbox{J}}{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{N} \cdot \mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{C} \cdot \mbox{s}^2} = \dfrac{\mbox{N} \cdot \mbox{m}} {\mbox{C}} = \dfrac{\mbox{J}}{\mbox{C}}

Josephson junction definition

Between 1990 and 1997 the volt was calibrated using the Josephson effect for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the the 18th General Conference on Weights and Measures. The following value for the Josephson constant is used:

K{J-90} = 2 e/ h = 0.4835979 GHz/µV.

This is typically used with an array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design). Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc, , and no correction terms are required in a practical implementation. However, as of July 2007, this is not the official BIPM definition of Volt.]

Water flow analogy

In the water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage difference is likened to water pressure difference–the difference determines how quickly 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 conductivity), and the pressure difference between the front end of the pipe and the exit (analogous to 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 (watts = amperes × volts).

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

Common voltages

A multimeter can be used to measure the voltage between two positions.
1.5 V C-cell batteries

Nominal voltages of familiar sources:

  • Nerve cell resting potential: around −75 mV
  • Single-cell, rechargeable NiMH or NiCd battery: 1.2 V
  • Mercury battery: 1.355 V
  • Single-cell, non-rechargeable alkaline battery (e.g., AAA, AA, C and D cells): 1.5 V
  • LiFePO4 rechargeable battery: 3.3 V
  • Lithium polymer rechargeable battery: 3.75 V (see Rechargeable battery#Table of rechargeable battery technologies)
  • Transistor-transistor logic/ CMOS (TTL) power supply: 5 V
  • PP3 battery: 9 V
  • Automobile electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running).
  • Household mains electricity: 230 V RMS in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see List of countries with mains power plugs, voltages and frequencies)
  • Trucks/ lorries: 24 V DC
  • Rapid transit third rail: 600–750 V (see List of current systems for electric rail traction)
  • High speed train overhead power lines: 25 kV RMS at 50 Hz, but see the list of current systems for electric rail traction and 25 kV at 60 Hz for exceptions.
  • High voltage electric power transmission lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005)
  • Lightning: Varies greatly, often around 100 MV.

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 as the unit for electromotive force. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.

The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favour of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.

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.

This SI unit is named after Alessandro Volta. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (V). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (volt), except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

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