Permeance

Permeance, in general, is the degree to which a material admits a flow of matter or energy. Permeance is usually represented by a curly capital P: $\mathcal{P}$.

Electromagnetism
In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger for large cross-sections of a material and smaller for smaller cross section lengths. This concept is analogous to electrical conductance in the electric circuit.

Magnetic permeance $\mathcal{P}$ is defined as the reciprocal of magnetic reluctance $\mathcal{R}$ (in analogy with the reciprocity between electric conductance and resistance): $$\mathcal{P} = \frac{1}{\mathcal{R}}$$

which can also be re-written: $$\mathcal{P} = \frac{\Phi_\mathrm{B}}{NI}$$

using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the definition of magnetomotive force (magnetic analogue of electromotive force): $$\mathcal{F} = \Phi_\mathrm{B} \mathcal{R} = NI$$

where:
 * $Φ_{B}$, magnetic flux,
 * $I$, current, in amperes,
 * $N$, winding number of, or count of turns in the electric coil.

Alternatively in terms of magnetic permeability (analogous to electric conductivity): $$\mathcal{P} = \frac{\mu A}{\ell}$$

where:
 * $μ$, permeability of material,
 * $A$, cross-sectional area,
 * ℓ, magnetic path length.

The SI unit of magnetic permeance is the henry (H), equivalently, webers per ampere.

Materials science
In materials science, permeance is the degree to which a material transmits another substance.

Electromagnetism

 * Properties of Magnetic Materials (units of magnetic permeance)

Material science

 * Bombaru, D., Jutras, R., and Patenaude, A., "Air Permeance of Building Materials". Summary report prepared by, AIR-INS Inc. for Canada Mortgage and Housing Corporation, Ottawa, 1988.