In physics, the ohm (symbol: Ξ©) is the SI unit of electrical impedance, named after German physicist Georg Simon Ohm. Electrical impedance is the measure of opposition to an electric current. An ohm is also a unit of electrical resistance, with the symbol R used in the field of electricity, named after Sir George Stokes. The reciprocal quantity to electrical resistance is called conductance G = 1/R and has its own SI unit siemens (S). An object with an impedance or resistance of one ohm resists an alternating current at a frequency of one hertz by producing voltage drops across itself equal in magnitude to that current but shifted in phase by 90 degrees.
Georg Ohm first used the term “ohm,” in 1827 in his publication Die galvanische Kette, mathematisch bearbeitet (The Galvanic Circuit Investigated Mathematically). In this work, he presented his experimental results showing that there was a linear relationship between potential difference and the resultant current flowing through simple metallic conductors such as wires for direct currents.