Which unit measures electrical resistance?

Resistance is measured in ohms, named after Georg Ohm, the German physicist who worked out the relationship between voltage, current and resistance in the 1820s. His law states that the current flowing through a conductor is proportional to the voltage across it and inversely proportional to its resistance, so raising resistance at fixed voltage cuts the current. It helps to picture water in a pipe: voltage is the pressure difference, current is the flow rate, and resistance is how narrow or obstructed the pipe is. A material's resistance depends on what it is made of, how long and thick the piece is, and its temperature, with most metals resisting more as they heat up. Resistance turns electrical energy into heat, which is a nuisance in power lines and the entire point in a kettle element or a toaster. Very high resistance materials are insulators, while some materials lose resistance entirely when cooled far enough.

The coloured bands printed on a small resistor encode its value in ohms and its tolerance.