Maxwell realized that Ampere's Law was incomplete for varying fields (like inside a charging capacitor).

• He introduced the Displacement Current ($I_d$): $I_d={ϵ}_0\frac{d{\Phi }_E}{dt}$
• This represents the effect of a changing electric field, which produces a magnetic field just like a real conduction current. This 'current' exists in the insulating gap of the capacitor, ensuring continuity of the total current.