The volt was sized to match a telegraph battery cell
When scientists fixed the volt in 1881, they chose a value close to the push of a Daniell cell, the workhorse battery of telegraph lines. Today the unit is realised with arrays of thousands of superconducting junctions driven by microwaves, and it spans everything from a 1.5 V AA cell to transmission lines above a million volts.
The volt measures electric potential difference, the quantity everyday speech calls voltage, and it honours Alessandro Volta. One volt exists between two points on a wire when a current of one ampere there dissipates one watt. Put another way, it gives one joule of energy to every coulomb of charge that crosses it, and it can also be written as amperes times ohms, following Ohm's law. In the popular water-pipe picture, voltage plays the role of pressure while current is the flow.
Volta's own contribution came in 1800, sparked by a disagreement with Luigi Galvani. His voltaic pile, a forerunner of the battery, delivered a steady current, and he found zinc paired with silver worked best. The name arrived later: Latimer Clark and Sir Charles Bright coined it in 1861, a formal definition came from the British Association for the Advancement of Science by 1873, and the International Electrical Congress adopted it for electromotive force in 1881, setting it at 10 to the power 8 cgs units because the old unit was awkwardly tiny.
Definitions kept shifting. An international volt tied to the Clark cell appeared in 1893 and was dropped in 1908. From 1990 a conventional volt relied on the Josephson effect, which converts frequency into voltage exactly, and the 2019 revision of the SI, effective 20 May 2019, gave the Josephson constant a fixed value. Laboratories typically chain several thousand to tens of thousands of junctions and drive them at 10 to 80 GHz; experiments show the result does not depend on device or material.
Real-world figures show the range. A rechargeable NiMH cell gives 1.2 V, an alkaline AA 1.5 V, a lithium coin cell 3 V. A so-called 12 V car battery strings six 2.1 V cells to produce 12.6 V. Mains supply runs at 230 V across Europe, Asia, Africa and Australia, metro third rails at 600 to 750 V, high-speed train wires at 25 kV, and the highest-voltage transmission line ever built reached 1.15 MV.
Source: Volt
