Josephson junction array chip developed by the National Bureau of Standards as a standard volt

SI derived unit 
Electric potential, electromotive force 
V 
Alessandro Volta 
kg·m·s·A 
The volt (symbol: V) is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. It is named after the Italian physicist Alessandro Volta (1745–1827).
Definition[edit]
One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, 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), watts per ampere (power per unit current, Joule's law), or joules per coulomb (energy per unit charge), which is also equivalent to electronvolts per elementary charge:
Josephson junction definition[edit]
The "conventional" volt, V, defined in 1988 by the 18th General Conference on Weights and Measures and in use from 1990, is implemented using the Josephson effect for exact frequencytovoltage conversion, combined with the caesium frequency standard. For the Josephson constant, K = 2e/h (where e is the elementary charge and h is the Planck constant), the "conventional" value K is used:
This standard is typically realized using a seriesconnected 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.
Waterflow analogy[edit]
In the waterflow analogy sometimes used to explain electric circuits by comparing them with waterfilled pipes, voltage (difference in electric potential) is likened to difference in water pressure. Current is proportional to the diameter of the pipe or the amount of water flowing at that pressure. A resistor would be a reduced diameter somewhere in the piping and a capacitor/inductor could be likened to a "U" shaped pipe where a higher water level on one side could store energy temporarily.
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[edit]
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:
History[edit]
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In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the socalled 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. By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad. In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force. They made the volt equal to 10 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. 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.
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.
See also[edit]
References[edit]
 ^ "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. Retrieved 20070729.
 ^ BIPM SI Brochure: Appendix 1, p. 144
 ^ Burroughs, Charles J.; Bent, Samuel P.; Harvey, Todd E.; Hamilton, Clark A. (19990601), "1 Volt DC Programmable Josephson Voltage Standard", IEEE Transactions on Applied Superconductivity, Institute of Electrical and Electronics Engineers (IEEE), 9 (3): 4145–4149, ISSN 10518223, doi:10.1109/77.783938, retrieved 20140627
 ^ Keller, Mark W (20080118), "Current status of the quantum metrology triangle" (PDF), Metrologia, 45 (1): 102–109, Bibcode:2008Metro..45..102K, ISSN 00261394, doi:10.1088/00261394/45/1/014,
 ^ Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; SchmidtNielsen, p. 484
 ^ Hill, Paul Horowitz; Winfield; Winfield, Hill (2015). The Art of Electronics (3. ed.). Cambridge [u.a.]: Cambridge Univ. Press. p. 689. ISBN 9780521809269.
 ^ SK Loo and Keith Keller (Aug 2004). "Singlecell Battery Discharge Characteristics Using the TPS61070 Boost Converter" (PDF). Texas Instruments.
 ^ 20122013 Model S Emergency Response Guide (PDF), retrieved 20170806
 ^ 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:
 Latimer Clark and Sir Charles Bright (1861) "On the formation of standards of electrical quantity and resistance," Report of the Thirtyfirst Meeting of the British Association for the Advancement of Science (Manchester, England: September 1861), section: Mathematics and Physics, pp. 3738.
 Latimer Clark and Sir Charles Bright (November 9, 1861) "Measurement of electrical quantities and resistance," The Electrician, 1 (1) : 3–4.
 ^ 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. 222225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 10 C.G.S. units of resistance ; the "volt" is approximately 10 C.G.S. units of electromotive force ; and the "farad" is approximately 1/10 of the C.G.S. unit of capacity."
 ^ (Anon.) (September 24, 1881) "The Electrical Congress," The Electrician, 7 : 297.
 ^ 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.
External links[edit]
Look up volt in Wiktionary, the free dictionary. 
Authority: International System of Units (BIPM)

