Ohm’s Law and the SI unit ohm are named in honor of Georg Simon Ohm (German physicist, 1787-1854). Ohm’s Law describes the relationship between current, voltage and resistance in an electric circuit. A conductor exhibits a resistance R of 1 ohm if, when applying a current U of 1 Volt, a current I of 1 ampere flows through this resistor: R = U/I. In physical terms, 1 ohm was formerly defined as the resistance of a mercury column with a length of 1.063 m and a diameter of 1 mm² at 0°C. Today, physicists use more exact but also more abstract and less easy to memorize definitions.