Ohm

The ohm is the SI derived unit of electrical resistance. It is symbolized by the Greek letter omega (Ω) and is named after the German physicist Georg Simon Ohm. One ohm is defined as the resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces a current of one ampere, meaning the conductor does not produce electromotive forces. Equivalently, one ohm equals one volt per ampere (Ω = V/A). The reciprocal of the ohm is the siemens (S), the unit of electrical conductance.

Georg Simon Ohm published Ohm's law in 1827, V = IR, describing how voltage, current, and resistance relate in an electrical circuit. The ohm was named in his honor and adopted as a standard SI unit for electrical resistance.

In practical use, resistances are measured in ohms with ohmmeters. Materials and components may have resistivity that depends on temperature and other conditions; in many devices the resistance is not constant, and the current–voltage relationship deviates from Ohm's law (non-ohmic behavior). The ohm remains a fundamental unit in electronics, circuit theory, and electrical engineering. In dimensional terms, the ohm has dimensions of M L^2 T^-3 I^-2.