# Reynolds number (for a magnetic field)

## Description

The magnetic Reynolds number is the magnetic analogue of the Reynolds number, a fundamental dimensionless group that occurs in magnetohydrodynamics. It gives an estimate of the relative effects of advection or induction of a magnetic field by the motion of a conducting medium, often a fluid, to magnetic diffusion.

The mechanism by which the motion of a conducting fluid generates a magnetic field is the subject of dynamo theory. When the magnetic Reynolds number is very large, however, diffusion and the dynamo are less of a concern, and in this case focus instead often rests on the influence of the magnetic field on the flow.

Related formulas## Variables

R_{m} | magnetic Reynolds number (dimensionless) (dimensionless) |

U | typical velocity scale of the flow (m/s) |

L | ypical length scale of the flow (m) |

η | magnetic diffusivity (m^{2}/s) |