Faraday' Law of Induction describes the fact that a spatially varying (and also possibly time-varying, depending on how a magnetic field varies in time) electric field always accompanies a time-varying magnetic field, while Faraday's law states that there is EMF (electromotive force, defined as electromagnetic work done on a unit charge when it has traveled one round of a conductive loop) on the conductive loop when the magnetic flux through the surface enclosed by the loop varies in time. Where: ? is a surface bounded by the closed contour ??, E is the electric field, B is the magnetic field. dl is an infinitesimal vector element of the contour ??, dA is an infinitesimal vector element of surface ?. If its direction is orthogonal to that surface patch, the magnitude is the area of an infinitesimal patch of surface. It can be applied to any smooth surface, comes in three sizes and two colors. Contact us if you require custom size or color. Proudly Made in the USA.