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Displacement current
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<p>In <a href="/facts/Electromagnetism/oAkmALHT">electromagnetism</a>, displacement current density is the quantity ∂D/∂<i>t</i> appearing in <a href="/facts/Maxwell%2527s_equations/LfJ1drHS">Maxwell's equations</a> that is defined in terms of the rate of change of D, the <a href="/facts/Electric_displacement_field/YERMcaRg">electric displacement field</a>. Displacement current density has the same units as electric current density, and it is a source of the <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> just as actual current is. However it is not an electric current of moving <a href="/facts/Electric_charge/xxsBfmlB">charges</a>, but a time-varying <a href="/facts/Electric_field/tt4MooBs">electric field</a>. In physical materials (as opposed to vacuum), there is also a contribution from the slight motion of charges bound in atoms, called <a href="/facts/Dielectric_polarization/PnVt9P2G">dielectric polarization</a>. </p><p>The idea was conceived by <a href="/facts/James_Clerk_Maxwell/jtYyze61">James Clerk Maxwell</a> in his 1861 paper <i><a href="https://books.google.com/books?id=v1YEAAAAYAAJ&pg=PA14">On Physical Lines of Force, Part III</a></i> in connection with the displacement of electric particles in a <a href="/facts/Dielectric/PnVt9P2G">dielectric</a> medium. Maxwell added displacement current to the <a href="/facts/Electric_current/Dc1xdnUX">electric current</a> term in <a href="/facts/Amp%25C3%25A8re%2527s_circuital_law/B6WVCpc6">Ampère's circuital law</a>. In his 1865 paper <a href="/facts/A_Dynamical_Theory_of_the_Electromagnetic_Field/zbe0J5rW">A Dynamical Theory of the Electromagnetic Field</a> Maxwell used this amended version of Ampère's circuital law to derive the <a href="/facts/Electromagnetic_wave_equation/qUYSPdeN">electromagnetic wave equation</a>. This derivation is now generally accepted as a historical landmark in physics by virtue of uniting electricity, magnetism and optics into one single unified theory. The displacement current term is now seen as a crucial addition that completed Maxwell's equations and is necessary to explain many phenomena, most particularly the existence of <a href="/facts/Electromagnetic_wave/Ynx7ujCW">electromagnetic waves</a>. </p>