tures; to be moisture-proof and inert to corrosive
agencies; and not to "dry out" or become brittle over a long period of
time.

_Space Utilization_. The utilization of the winding space in an
electromagnet is an important factor in design, since obviously the
copper or other conductor is the only part of the winding that is
effective in setting up magnetizing force. The space occupied by the
insulation is, in this sense, waste space. An ideally perfect winding
may be conceived as one in which the space is all occupied by wire;
and this would necessarily involve the conception of wire of square
cross-section and insulation of infinite thinness. In such a winding
there would be no waste of space and a maximum amount of metal
employed as a conductor. Of course, such a condition is not possible
to attain and in practice some insulating material must be introduced
between the layers of wire and between the adjacent convolutions of
wire. The ratio of the space occupied by the conductor to the total
space occupied by the winding, that is, by the conductor and the
insulation, is called the _coefficient of space utilization of the
coil_. For the ideal coil just conceived the coefficient of space
utilization would be 1. Ordinarily the coefficient of space
utilization is greater for coarse wire than for fine wire, since
obviously the ratio of the diameter of the wire to the thickness of
the insulation increases as the size of the wire grows larger.

The chief advantage of enamel insulation for magnet wire is its
thinness, and the high coefficient of space utilization which may be
secured by its use. In good enamel wire the insulation will average
about one-quarter the thickness of the standard single silk
insulation, and the dielectric strength is equal or greater. Where
economy of winding space is desirable the advantages of this may
readily be seen. For instance, in a given coil wound with No. 36
single silk wire about one-half of the winding space is taken up with
the insulation, whereas when the same coil is wound with No. 36
enameled wire only about one-fifth of the winding space is taken up by
the insulation. Thus the coefficient of space utilization is increased
from .50 to .80. The practical result of this is that, in the case of
any given winding space where No. 36 wire is used, about 60 per cent
more turns can be put on with enameled wire than with single silk
insulation, and of course this ratio greatly increases whe