There exist some applications where it is more economical to produce a coil without a magnetic core. A low inductance but high current inductor could be one example. Coil turns are wound on a supporting mandrel and bonded together into a rigid coil or wound on an insulated form which gives the coil support such as a bobbin (or spool), a tube, or a non-magnetic toroidal form. Such coils may be referred to as air core coils. The relative permeability of air and most insulators is one. The permeability of air is constant. It does not change with temperature, unless conditions induce formation of corona and/or plasma. Coils wound on insulating forms may have slight inductance changes due to polarization effects on the molecules of the coil form.