Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter . The term was coined in September, 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity. In SI units, permeability is measured in the henry per metre (H m-1), or newton per ampere squared (N A-2). The constant value 0 is known as the magnetic constant or the permeability of free space, and has the exact (defined)[1] value 0 = 4 ×10 7 N·A 2.