Ohm

The ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and current, expressed in Ohm's Law.

By definition from Ohm's Law, a device has a resistance of 1 ohm if a voltage of 1 volt will cause a current of 1 ampere to flow (R = V/I). This can also be represented in SI base units; a device that dissipates one watt of power with one ampere of current flowing through it has a resistance of one ohm (R = P/I²).

One thousand ohms is called a kilohm (not kilo-ohm), and abbreviated kΩ. One million ohms is called a megohm (not mega-ohm), and abbreviated MΩ. Other orders of magnitude follow standard SI conventions.

A measurement in ohms is the reciprocal of a measurement in siemens (also called mho, the anagram of ohm), the SI unit of electrical conductance. Note that 'siemens' is both singular and plural.

Ohms are also used to measure impedance and reactance for complex resistance.

See also

• Ohm's Law