r covering magnet wire is of silk or cotton. Of these, silk is by
far the better material for all ordinary purposes, since it has a much
higher insulating property than cotton, and is very much thinner.
Cotton, however, is largely employed, particularly in the larger sizes
of magnet wire. Both of these materials possess the disadvantage of
being hygroscopic, that is, of readily absorbing moisture. This
disadvantage is overcome in many cases by saturating the coil after it
is wound in some melted insulating compound, such as wax or varnish or
asphaltum, which will solidify on cooling. Where the coils are to be
so saturated the best practice is to place them in a vacuum chamber
and exhaust the air, after which the hot insulating compound is
admitted and is thus drawn into the innermost recesses of the winding
space.

Silk-insulated wire, as regularly produced, has either one or two
layers of silk. This is referred to commercially as single silk wire
or as double silk wire. The single silk has a single layer of silk
fibers wrapped about it, while the double silk has a double layer, the
two layers being put on in reverse direction. The same holds true of
cotton insulated wire. Frequently, also, there is a combination of the
two, consisting of a single or a double wrapping of silk next to the
wire with an outer wrapping of cotton. Where this is done the cotton
serves principally as a mechanical protection for the silk, the
principal insulating properties residing in the silk.

_Enamel_. A later development in the insulation of magnet wire has
resulted in the so-called enamel wire. In this, instead of coating the
wire with some fibrous material such as silk or cotton, the wire is
heated and run through a bath of fluid insulating material or liquid
enamel, which adheres to the wire in a very thin coating. The wire is
then run through baking ovens, so that the enamel is baked on. This
process is repeated several times so that a number of these thin
layers of the enamel are laid on and baked in succession.

The characteristics sought in good enamel insulation for magnet wire
may be thus briefly set forth: It is desirable for the insulation to
possess the highest insulating qualities; to have a glossy, flawless
surface; to be hard without being brittle; to adhere tenaciously and
stand all reasonable handling without cracking or flaking; to have a
coefficient of elasticity greater than the wire itself; to withstand
high tempera