Volt
The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force. It is named in honor of the Lombard physicist Alessandro Volta  (17451827), who invented the voltaic pile, the first modern chemical battery.

# Definition
The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m2  kg  s-3  A-1, which can be equally represented as one joule of energy per coulomb of charge, J/C.

## 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

# 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.  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 resistance) and the pressure difference between the front end of the pipe and the exit (potential difference or 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 (amperes x volts = watts).
The relationship between voltage and current (in ohmic devices) is defined by Ohm's Law.

# Common voltages
Nominal voltages of familiar sources:
- Nerve cell action 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
- Lithium polymer rechargeable battery: 3.75 V
- Transistor-transistor logic/CMOS (TTL) power supply: 5 V
- PP3 battery: 9 V
- Automobile electrical system: "12 V", about 11.8 V discharged, 12.8 V charged, and 13.8-14.4 V while charging (vehicle running).
- Household mains electricity: 240 V RMS in Australia, 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)
- Rapid transit third rail: 600 to 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 for exceptions.
- High voltage electric power transmission lines: 110 kV RMS and up (1150 kV 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 for electromotive force. At that time, 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. The United States used a design called the Weston cell from 1905 to 1972.