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The total magnetic induction or flux corresponds to the current of electricity (practically measured in amperes); the induction or flux density B to the density of the current (number of amperes to the square centimetre of section); the magnetic permeability to the specific electric conductivity; and the line integral of the magnetic force, sometimes called the magnetomotive force, to the electro-motive force in the circuit.

This sum, or integral, is called the " line integral of electric force" or the electromotive force (E.M.F.) along this line.

The fundamental fact connecting electric currents and magnetic fields is that the line integral of magnetic force taken once round a conductor conveying an electric current is equal to 4 7r-times the surface integral of the current density, or to 4 7r-times the total current flowing through the closed line round which the integral is taken (see Electrokinetics).