In classical electromagnetism, Ampère's circuital law, discovered by AndréMarie Ampère in 1826,^{[1]} relates the integrated magnetic field around a closed loop to the electric current passing through the loop. Maxwell derived it again electrodynamically in his 1861 paper On Physical Lines of Force and it is now one of the Maxwell equations, which form the basis of classical electromagnetism.
Contents 
In its historically original form, Ampère's Circuital Law relates the magnetic field to its electric current source. The law can be written in two forms, the "integral form" and the "differential form". The forms are equivalent, and related by the Kelvin–Stokes theorem.
In SI units (the version in cgs units is in a later section), the "integral form" of the original Ampère's circuital law is:^{[2]}^{[3]}
or equivalently,
where
There are a number of ambiguities in the above definitions that warrant elaboration.
First, three of these terms are associated with sign ambiguities: the line integral could go around the loop in either direction (clockwise or counterclockwise); the vector area dS could point in either of the two directions normal to the surface; and I_{enc} is the net current passing through the surface S, meaning the current passing through in one direction, minus the current in the other direction—but either direction could be chosen as positive. These ambiguities are resolved by the righthand rule: With the palm of the righthand toward the area of integration, and the indexfinger pointing along the direction of lineintegration, the outstretched thumb points in the direction that must be chosen for the vector area dS. Also the current passing in the same direction as dS must be counted as positive. The right hand grip rule can also be used to determine the signs.
Second, there are infinitely many possible surfaces S that have the curve C as their border. (Imagine a soap film on a wire loop, which can be deformed by blowing gently at it.) Which of those surfaces is to be chosen? If the loop does not lie in a single plane, for example, there is no one obvious choice. The answer is that it does not matter; it can be proven that any surface with boundary C can be chosen.
By the Kelvin–Stokes theorem, this equation can also be written in a "differential form". Again, this equation only applies in the case where the electric field is constant in time; see below for the more general form. In SI units, the equation states:
where
The electric current that arises in the simplest textbook situations would be classified as "free current"—for example, the current that passes through a wire or battery. In contrast, "bound current" arises in the context of bulk materials that can be magnetized and/or polarized. (All materials can to some extent.)
When a material is magnetized (for example, by placing it in an external magnetic field), the electrons remain bound to their respective atoms, but behave as if they were orbiting the nucleus in a particular direction, creating a microscopic current. When the currents from all these atoms are put together, they create the same effect as a macroscopic current, circulating perpetually around the magnetized object. This magnetization current J_{M} is one contribution to "bound current".
The other source of bound current is bound charge. When an electric field is applied, the positive and negative bound charges can separate over atomic distances in polarizable materials, and when the bound charges move, the polarization changes, creating another contribution to the "bound current", the polarization current J_{P}.
The total current density J due to free and bound charges is then:
with J_{f} the "free" or "conduction" current density.
All current is fundamentally the same, microscopically. Nevertheless, there are often practical reasons for wanting to treat bound current differently from free current. For example, the bound current usually originates over atomic dimensions, and one may wish to take advantage of a simpler theory intended for larger dimensions. The result is that the more microscopic Ampère's law, expressed in terms of B and the microscopic current (which includes free, magnetization and polarization currents), is sometimes put into the equivalent form below in terms of H and the free current only. For a detailed definition of free current and bound current, and the proof that the two formulations are equivalent, see the "proof" section below.
There are two important issues regarding Ampère's law that require closer scrutiny. First, there is an issue regarding the continuity equation for electrical charge. There is a theorem in vector calculus that states the divergence of a curl must always be zero. Hence ∇·(∇×B) = 0 and so the original Ampère's law implies that ∇·J = 0. But in general ∇·J = −∂ρ/∂t, which is nonzero for a timevarying charge density. An example occurs in a capacitor circuit where timevarying charge densities exist on the plates.^{[4]}^{[5]}^{[6]}^{[7]}^{[8]}
Second, there is an issue regarding the propagation of electromagnetic waves. For example, in free space, where J = 0, Ampère's law implies that ∇×B = 0, but instead ∇×B = −(1/c^{2}) ∂E/∂t.
To treat these situations, the contribution of displacement current must be added to the "free current" term in Ampère's law.
James Clerk Maxwell conceived of displacement current as a polarization current in the dielectric vortex sea, which he used to model the magnetic field hydrodynamically and mechanically.^{[9]} He added this displacement current to Ampère's circuital law at equation (112) in his 1861 paper On Physical Lines of Force.^{[10]}
In free space, the displacement current is related to the time rate of change of electric field.
In a dielectric the above contribution to displacement current is present too, but a major contribution to the displacement current is related to the polarization of the individual molecules of the dielectric material. Even though charges cannot flow freely in a dielectric, the charges in molecules can move a little under the influence of an electric field. The positive and negative charges in molecules separate under the applied field, causing an increase in the state of polarization, expressed as the polarization density P. A changing state of polarization is equivalent to a current.
Both contributions to the displacement current are combined by defining the displacement current as:^{[4]}
where the electric displacement field is defined as:
where ε_{0} is the electric constant and P is the polarization density. Substituting this form for D in the expression for displacement current, it has two components:
The first term on the right hand side is present everywhere, even in a vacuum. It doesn't involve any actual movement of charge, but it nevertheless has an associated magnetic field, as if it were an actual current. Some authors apply the name displacement current to only this contribution.^{[11]}
The second term on the right hand side is the displacement current as originally conceived by Maxwell, associated with the polarization of the individual molecules of the dielectric material.
Maxwell's original explanation for displacement current focused upon the situation that occurs in dielectric media. In the modern postaether era, the concept has been extended to apply to situations with no material media present, for example, to the vacuum between the plates of a charging vacuum capacitor. The displacement current is justified today because it serves several requirements of an electromagnetic theory: correct prediction of magnetic fields in regions where no free current flows; prediction of wave propagation of electromagnetic fields; and conservation of electric charge in cases where charge density is timevarying. For greater discussion see Displacement current.
Next Ampère's equation is extended by including the polarization current, thereby remedying the limited applicability of the original Ampère's circuital law.
Treating free charges separately from bound charges, Ampère's equation including Maxwell's correction in terms of the Hfield is (the Hfield is used because it includes the magnetization currents, so J_{M} does not appear explicitly, see Hfield and also Note):^{[12]}
(integral form), where H is the magnetic H field (also called "auxiliary magnetic field", "magnetic field intensity", or just "magnetic field", D is the electric displacement field, and J_{f} is the enclosed conduction current or free current density. In differential form,
On the other hand, treating all charges on the same footing (disregarding whether they are bound or free charges), the generalized Ampère's equation (also called the Maxwell–Ampère equation) is (see the "proof" section below):
in integral form. In differential form,
In both forms J includes magnetization current density^{[13]} as well as conduction and polarization current densities. That is, the current density on the right side of the Ampère–Maxwell equation is:
where current density J_{D} is the displacement current, and J is the current density contribution actually due to movement of charges, both free and bound. Because ∇ · D = ρ, the charge continuity issue with Ampère's original formulation is no longer a problem.^{[14]} Because of the term in ε_{0}∂E / ∂t, wave propagation in free space now is possible.
With the addition of the displacement current, Maxwell was able to hypothesize (correctly) that light was a form of electromagnetic wave. See electromagnetic wave equation for a discussion of this important discovery.
Proof that the formulations of Ampère's law in terms of free current are equivalent to the formulations involving total current. 

In this proof, we will show that the equation
is equivalent to the equation Note that we're only dealing with the differential forms, not the integral forms, but that is sufficient since the differential and integral forms are equivalent in each case, by the Kelvin–Stokes theorem. We introduce the polarization density P, which has the following relation to E and D: Next, we introduce the magnetization density M, which has the following relation to B and H: and the following relation to the bound current: where is called the magnetization current density, and is the polarization current density. Taking the equation for B: Consequently, referring to the definition of the bound current: as was to be shown. 
In cgs units, the integral form of the equation, including Maxwell's correction, reads
where c is the speed of light.
The differential form of the equation (again, including Maxwell's correction) is

