�y��vlog

To obtain an exclusion, each steel user must fill out a lengthy Excel spreadsheet, detailing for government bureaucrats its product needs, including the chemical composition, dimensions, tensile strength, coating, magnetic permeability and global and local “ductility.”

For a material defined by its electric permittivity, ε, and magnetic permeability, , the Fresnel reflection coefficient for the s polarization has the form where is the wave impedance for material j, and θi and t are the incident and transmitted angle of the wave with respect to the surface normal.

But if it were found that, for instance, the effect produced depended more on the specific inductive capacity than on the magnetic permeability of the medium—which would certainly be the case for speeds incomparably lower than that of light—then I believe I would be justified in saying that the effect produced was more of an electrostatic nature.

But for this to work with visible light, the electric permittivity and magnetic permeability -- which describe a material's response to electric and magnetic fields -- of the material used would probably need to vary wildly throughout the cloak.

In 2008, Pendry and others demonstrated the beginnings of a cloak for static magnetic fields, the simplicity of which resided in the need to vary only the cloak's magnetic permeability, and not its electric permittivity.