A measure of the relative ease with which a magnetic field can be established in a medium, with a free field being the constant, or zero reference (1.257 x 10 to the 6th power). For example, many metals have higher permeability than materials such as plastic. Iron has particularly high permeability and is sometimes used in inductors and transformers for this reason (though there are other materials that are better). High permeability materials are also used in places like magnetic tape heads, and magnetic shields. Permeability is measured in henrys per meter (H/m) and is represented by the Greek letter u (mu).