Reluctance

Reluctance is the magnetic circuit equivalent of resistance in a DC electrical circuit. Reluctance is defined as the ratio of the 'magnetomotive force' over the magnetic flux, in the same way that resistance is defined as the ratio of EMF over current.

Suppose 10 Amps AC in the primary winding of a transformer generates a changing magnetic flux in the core that induces one Amp AC in the secondary. When paper is inserted into the gap between the cores the secondary current drops to 0.5 Amps AC. The reluctance has doubled.