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