Ohm

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This article is about the SI derived unit. For other meanings, see Ohm (disambiguation).

A multimeter can be used to measure resistance in ohms. It can also be used to measure capacitance, voltage, current, and other electrical characteristics.

Several resistors. Their resistance, in ohms, is marked using a color code.

The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Ohm.

1 Definition
2 Conversions
3 References and notes
4 See also
5 External links

Definition

The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.^[1]

$\Omega = \dfrac{\mbox{V}}{\mbox{A}} = \dfrac{\mbox{m}^2 \cdot \mbox{kg}}{\mbox{s}^{3} \cdot \mbox{A}^2}$

Conversions

A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance. Note that 'siemens' is both singular and plural. The non-SI unit, the mho (ohm written backwards), is equivalent to siemens but is mostly obsolete and rarely used.
Ohms to watts: The power dissipated by a resistor may be calculated using resistance and voltage. The formula is a combination of Ohm's law and Joule's law:

$P={V^2 \over R}$

where P is the power in watts, R is the resistance in ohms and V is the voltage across the resistor.

This method is not reliable for determining the power of an incandescent light bulb, resistance heater or electric short since all these devices operate at high temperatures and the resistance measured using a meter will not represent the operating resistance. For these conditions instead multiply V by current in amperes to get power in watts.