The fundamental force vector in magnetism; the magnetic analogue of the electric field E. Both a magnet and a current-carrying coil exert forces on other coils or magnets. The magnetic flux density produced by such magnets or coils is a vector quantity and lines of flux can be drawn whose direction at any point is the direction of magnetic flux density. The value of B is given by the number of lines of flux per unit area and is expressed by the equation

where dF is the force exerted due to B on an element of length ds of wire carrying a current I. This defines the unit of magnetic flux density as that which exerts a force of one newton on a wire of length one metre carrying a current of one ampere.