 x Volt Encyclopedia
The volt is the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

derived unit
SI derived unit
The International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...

for electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

, electric potential difference, and electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

. The volt is named in honor of the Italian physicist Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

(1745–1827), who invented the voltaic pile
Voltaic pile
A voltaic pile is a set of individual Galvanic cells placed in series. The voltaic pile, invented by Alessandro Volta in 1800, was the first electric battery...

, possibly the first chemical battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

.

## Definition

A single volt is defined as the difference in electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

across a wire
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

when an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

of one ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

dissipates one watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

of power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

. It is also equal to the potential difference between two points 1 meter apart in an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

of 1 newton
Newton
Newton is a contraction of "New Town", the name of many places in English-speaking countries. It may refer to:-People:* Isaac Newton , English mathematician, philosopher and scientist* Newton , lists of notable people with the surname...

per coulomb. Additionally, it is the potential difference between two points that will impart one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

per coulomb of charge that passes through it. It can be expressed in terms of SI units as follows: It can also be written using only the SI base units m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

, kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

, s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, and A
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

as: ### Josephson junction definition

Between 1990 and 1997, the volt was calibrated using the Josephson effect
Josephson effect
The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...

for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the 18th General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

. The following value for the Josephson constant is used:
K{J-90} = 2e
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...

/h
Planck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...

= 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
Hydraulic analogy
The electronic–hydraulic analogy is the most widely used analogy for "electron fluid" in a metal conductor. Since electric current is invisible and the processes at play in electronics are often difficult to demonstrate, the various electronic components are represented by hydraulic...

sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

difference is likened to water pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

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
Volumetric flow rate
The volumetric flow rate in fluid dynamics and hydrometry, is the volume of fluid which passes through a given surface per unit time...

). 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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

). 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
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...

.

## Common voltages Nominal voltages of familiar sources:
• Nerve cell resting potential
Resting potential
The relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential....

: around −75 mV
• Single-cell, rechargeable NiMH
Nickel metal hydride battery
A nickel–metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel–cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide...

or NiCd
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....

battery: 1.2 V
• Mercury battery
Mercury battery
A mercury battery is a non-rechargeable electrochemical battery, a primary cell. Due to the content of mercury, and the resulting environmental concerns, the sale of mercury batteries is banned in many countries. Both ANSI and IEC have withdrawn standards for mercury batteries...

: 1.355 V
• Single-cell, non-rechargeable alkaline battery
Alkaline battery
Alkaline batteries are a type of primary batteries dependent upon the reaction between zinc and manganese dioxide . A rechargeable alkaline battery allows reuse of specially designed cells....

(e.g., AAA, AA, C and D cells): 1.5 V
• LiFePO4
Lithium iron phosphate battery
The lithium iron phosphate battery, also called LFP battery, is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO4 as a cathode material.-History:...

rechargeable battery: 3.3 V
• Lithium polymer rechargeable battery: 3.75 V (see Rechargeable battery#Table of rechargeable battery technologies)
• Transistor-transistor logic
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...

/CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

(TTL) power supply: 5 V
• PP3 battery
PP3 battery
A nine-volt battery, the most common of which is designated a PP3 battery, is shaped as a rounded rectangular prism. 9-volt batteries are commonly used in pocket transistor radios, smoke detectors, carbon monoxide alarms, guitar effect units, and radio-controlled vehicle controllers...

: 9 V
• Automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

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
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

: 230 V RMS
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...

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)
• Truck
Truck
A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

s/lorries: 24 V DC
• Rapid transit
Rapid transit
A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

third rail
Third rail
A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...

: 600–750 V (see List of current systems for electric rail traction)
• High-speed train overhead power lines: 25 kV RMS at 50 Hz
25 kV AC
The 25 kV Alternating current railway electrification system is commonly used in railways worldwide, especially for high-speed rail.-Overview:This electrification system is ideal for railways that cover long distances and/or carry heavy traffic...

, but see the list of current systems for electric rail traction and 25 kV at 60 Hz for exceptions.
• High-voltage electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005)
• Lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

: Varies greatly, often around 100 MV.

Note: Where RMS (root mean square
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...

) is stated above, the peak voltage is 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
Luigi Galvani
Luigi Aloisio Galvani was an Italian physician and physicist who lived and died in Bologna. In 1791, he discovered that the muscles of dead frogs legs twitched when struck by a spark...

, Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

developed the so-called Voltaic pile
Voltaic pile
A voltaic pile is a set of individual Galvanic cells placed in series. The voltaic pile, invented by Alessandro Volta in 1800, was the first electric battery...

, a forerunner of the battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity is zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

and silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...

(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
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

dissipates one watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

of power.

The international volt was defined in 1893 as 1/1.434 of the emf
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

of a Clark cell
Clark cell
The Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell that produces a highly stable voltage usable as a laboratory standard.-Chemistry:...

. This definition was abandoned in 1908 in favor of a definition based on the international ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

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
Weston cell
The Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters...

. The United States used a design called the Weston cell
Weston cell
The Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters...

from 1905 to 1972.

• Ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

• Electric potential difference
• Rail traction voltage

• Ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

• SI electromagnetism units
• SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

for unit prefixes

• Voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

• Voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...

• Watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:... 