ohm
Resistanceexact by definition
The ohm is the SI unit of electrical resistance, defined as the resistance through which one volt drives one ampere. As a coherent derived unit it equals one volt per ampere exactly, so its relation to the SI base units is exact by definition rather than measured. Resistance is the ratio of voltage to current for a component, not a property the component carries on its own until you say at what temperature.
| 1 Ω | 1 Ω |
Named for Georg Simon Ohm, whose 1827 book established the proportionality between applied voltage and resulting current that now carries his name.
About the ohm
Ohm's law, \(V = IR\), is not a law of nature in the way conservation of energy is. It is a description of a class of materials, called ohmic, whose resistance stays roughly constant as voltage changes. Metals are reasonably ohmic at fixed temperature; diodes, lamp filaments, thermistors and electrolytes are not, and their resistance at one operating point tells you little about another. When Ohm published in 1827 the result was received badly, partly because it looked too simple and partly because his experimental method was not yet trusted.
Since 2019 the ohm is realised through the quantum Hall effect, where a two-dimensional electron gas in a strong magnetic field shows plateaux of resistance at exactly \(R_K/n\), with the von Klitzing constant \(R_K = h/e^2\). Both \(h\) and \(e\) are now fixed numbers, so \(R_K\) is exactly 25812.80745 ohms and any laboratory with the apparatus can reproduce the ohm from first principles. The practical consequence for everyone else is that a calibrated resistor traces back to a physical constant rather than to a wire artefact kept in a vault.