It is to be understood that in Table III the wire referred to is bare wire and is of pure copper.  It is not commercially practicable to use absolutely pure copper, and the ordinary magnet wire has a conductivity equal to about 98 per cent of that of pure copper.  The figures given in this table are sufficiently accurate for all ordinary practical purposes.

Silk and Cotton Insulation.  The insulating material usually employed for 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 temperatures; to be moisture-proof and inert to corrosive agencies; and not to “dry out” or become brittle over a long period of time.