 These effects can be combined in a partial differential equation for the magnetic field called the "magnetic induction equation",

where is the velocity of the fluid; is the magnetic B-field; and is the magnetic diffusivity, which is inversely proportional to the product of the electrical conductivity and the permeability .The term is the time derivative of the field; is the Laplace operator and is the curl operator.The first term on the right hand side of the induction equation is a diffusion term.