Current, Displacement

In establishing the theory of the electromagnetic field, J. C. Maxwell proposed a hypothesis— later confirmed by experiment—stating that a magnetic field is created not only by the motion of charges (conduction current, or simply current) but also by any change in the electric field over time. Maxwell designated as “displacement current” a quantity equal to the change over time t of the induction D—or, more accurately, the quantity (∂D/∂t)/4π. The rotational magnetic field is determined by the total current j = j_cond + (∂D/∂t)/4π, where j_cond is the conduction current density. A displacement current creates a magnetic field according to the same law that applies to a conduction current; it is for this reason that the designation “current” is applied to the quantity (∂D/∂t)/4π. (SeeMAXWELL’S EQUATIONS.)