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Josephson voltage standard chip developed by the National Bureau of Standards as a standard volt
General information
Unit system SI
Unit of electric potential, electromotive force
Named after Alessandro Volta
SI base units kgm 2s −3A −1

The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). [1] It is named after the Italian physicist Alessandro Volta (1745–1827).



One volt is defined as the electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. [2] Equivalently, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it. It can be expressed in terms of SI base units (m, kg, s, and A) as

It can also be expressed as amperes times ohms (current times resistance, Ohm's law), webers per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge), which is also equivalent to electronvolts per elementary charge:

Josephson junction definition

Historically, the "conventional" volt, V90, defined in 1987 by the 18th General Conference on Weights and Measures [3] and in use from 1990 to 2019, was implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the caesium frequency standard.

For the Josephson constant, KJ = 2e/h (where e is the elementary charge and h is the Planck constant), a "conventional" value KJ-90 = 0.4835979 GHz/μV was used for the purpose of defining the volt. As a consequence of the 2019 redefinition of SI base units, as of 2019 the Josephson constant has an exact value of KJ = 483597.84841698... GHz/V, which replaced the conventional value KJ-90.

This standard is typically realized using a series-connected array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design). [4] 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. [5]

Water-flow analogy

In the water-flow analogy , sometimes used to explain electric circuits by comparing them with water-filled pipes, voltage (difference in electric potential) is likened to difference in water pressure, while current is proportional to the amount of water flowing. A resistor would be a reduced diameter somewhere in the piping or something akin to a radiator offering resistance to flow. Perhaps a capacitor could be likened to a U bend where a higher water level can store energy and build up a head of pressure.

Perhaps an inductor could be likened to a fly wheel apparatus.

The relationship between voltage and current is defined (in ohmic devices like resistors) by Ohm's law. Ohm's Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.

Common voltages

A multimeter can be used to measure the voltage between two positions. Electronic multi meter.jpg
A multimeter can be used to measure the voltage between two positions.
1.5 V C-cell batteries BateriaR14.jpg
1.5 V C-cell batteries

The voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell (see Galvanic cell § Cell voltage). Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can usually be constructed to any voltage in a range of feasibility.

Nominal voltages of familiar sources:


Alessandro Volta Alessandro Volta.jpeg
Alessandro Volta
Group photograph of Hermann Helmholtz, his wife (seated) and academic friends Hugo Kronecker (left), Thomas Corwin Mendenhall (right), Henry Villard (center) during the International Electrical Congress PSM V85 D521 Group photograph of herman helmholtz and academic friends.png
Group photograph of Hermann Helmholtz, his wife (seated) and academic friends Hugo Kronecker (left), Thomas Corwin Mendenhall (right), Henry Villard (center) during the International Electrical Congress

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 1861, Latimer Clark and Sir Charles Bright coined the name "volt" for the unit of resistance. [11] By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad. [12] In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force. [13] They made the volt equal to 108 cgs units of voltage, the cgs system at the time being the customary system of units in science. They chose such a ratio because the cgs unit of voltage is inconveniently small and one volt in this definition is approximately the emf of a Daniell cell, the standard source of voltage in the telegraph systems of the day. [14] 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 favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948. [15]

A redefinition of SI base units, including defining the value of the elementary charge, took effect on 20 May 2019. [16]

See also

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  1. "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. Archived from the original on 2007-06-18. Retrieved 2007-07-29.
  2. BIPM SI Brochure: Appendix 1, p. 144.
  3. "Resolutions of the CGPM: 18th meeting (12–15 October 1987)".
  4. Burroughs, Charles J.; Bent, Samuel P.; Harvey, Todd E.; Hamilton, Clark A. (1999-06-01), "1 Volt DC Programmable Josephson Voltage Standard", IEEE Transactions on Applied Superconductivity, Institute of Electrical and Electronics Engineers (IEEE), 9 (3): 4145–4149, Bibcode:1999ITAS....9.4145B, doi:10.1109/77.783938, ISSN   1051-8223, S2CID   12970127
  5. Keller, Mark W. (2008-01-18), "Current status of the quantum metrology triangle" (PDF), Metrologia, 45 (1): 102–109, Bibcode:2008Metro..45..102K, doi:10.1088/0026-1394/45/1/014, ISSN   0026-1394, S2CID   122008182, archived from the original (PDF) on 2010-05-27, retrieved 2010-04-11, Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that KJ and RK are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright.
  6. Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484.
  7. Hill, Paul Horowitz; Winfield; Winfield, Hill (2015). The Art of Electronics (3. ed.). Cambridge [u.a.]: Cambridge Univ. Press. p. 689. ISBN   978-0-521-809269.
  8. SK Loo and Keith Keller (Aug 2004). "Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter" (PDF). Texas Instruments.
  9. "World's Biggest Ultra-High Voltage Line Powers Up Across China". www.bloomberg.com. 1 January 2019. Retrieved 7 January 2020.
  10. Paul H. Risk (26 Jun 2013). "Lightning – High-Voltage Nature". RiskVA.
  11. As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:
  12. Sir W. Thomson, et al. (1873) "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units," Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222-225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 109 C.G.S. units of resistance ; the "volt" is approximately 108 C.G.S. units of electromotive force ; and the "farad" is approximately 1/109 of the C.G.S. unit of capacity."
  13. (Anon.) (September 24, 1881) "The Electrical Congress," The Electrician, 7 : 297.
  14. Hamer, Walter J. (January 15, 1965). Standard Cells: Their Construction, Maintenance, and Characteristics (PDF). National Bureau of Standards Monograph #84. US National Bureau of Standards.
  15. "Revised Values for Electrical Units" (PDF). Bell Laboratories Record. XXV (12): 441. December 1947.
  16. Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018) (PDF), archived from the original (PDF) on 2018-04-29, retrieved 2018-11-02