In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid. Rotation in the outer core is supplied by the Coriolis effect caused by the rotation of the Earth. The Coriolis force tends to organize fluid motions and electric currents into columns (also see Taylor columns) aligned with the rotation axis. Induction or generation of magnetic field is described by the induction equation : ∂ ∂ B ∂ ∂ t = η η ∇ ∇ 2 B + ∇ ∇ × × ( u × × B ) where u is velocity, B is magnetic field, t is time, and η η = 1 / ( σ σ μ μ ) is the magnetic diffusivity with σ σ electrical conductivity and μ μ permeability. The ratio of the second term on the right hand side to the first term gives the magnetic Reynolds number, a dimensionless ratio of advection of magnetic field to diffusion.