Ohm

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The ohm is the SI unit of electrical resistance. 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 in Ohm's Law, 1 ohm equals 1 volt divided by 1 ampere. In other words, a device has a resistance of 1 ohm if a voltage of 1 volt will cause a current of 1 ampere to flow.

A thousand ohms is called a kilohm (not kilo-ohm). A million ohms is called a megohm (not mega-ohm).

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.