Volt

The volt is the standard unit of voltage. It is defined as the electric potential difference required to move one ampere of current through a conductor with one ohm of resistance. According to Ohm's law, one watt of power is used, which is released as heat and warms the conductor.

An analogy to voltage is water pressure. Water will only move between two points if there is a difference in pressure between them. A volt is basically a unit amount of extra force.

History of the Volt

The volt was defined at an international conference in 1881, and is named after Alessandro Volta (1745-1827). Volta invented the voltaic pile that was the forerunner of the common cell battery. Before then, units called abvolt and ohma were used to measure electromotive force. The symbol for volts is V, and is always written in uppercase.

Measuring Volts

Volts are measured with a voltmeter or multimeter. Two probes are placed at different points in the circuit and the potential difference between these points is shown on the screen.

Direct current usually has a steady voltage which is easy to measure. Alternating current has a voltage that changes constantly, so a mean value called Root-Mean-Square (RMS) voltage is used. It is calculated with a complex mathematical formula, which is done automatically on digital meters.

Megavolt, Kilovolt and Millivolt

A megavolt (MV) is one million volts, a kilovolt (kV) is one thousand volts, and a millivolt (mV) is one thousandth of a volt. Everyday voltages range from nerve signals with a few millivolts, to lightning with megavolts. Powerpoint voltages range from 120V to 240V, but powerlines can have many kilovolts in them.

A common misconception is that high voltage is lethal. It is the current flowing through the body that does the damage, by burning tissue and disrupting signals. A static shock can have thousands of volts, but does no damage because it has very little current.