As most wires used are perfectly round, a circular measure is used to express the area--the circular mil. One circular mil is the area of a circle whose diameter is 1 mil. The term mil means one-thousandth of an inch (1/1,000 or 0.001 inch = 1 mil).
If the diameter of a wire measures 20.10 mils then it has a cross-sectional area of 404.0 circular mils, the area being obtained by squaring the diameter of the wire in mils (multiplying it by itself).
When purchasing spools of copper wire you will find the number of the wire marked on the spool in most cases, but occasionally you may find it necessary to measure the size of the wire yourself. In that case you can use a micrometer such as every tool maker has, one of which is shown in Fig. 14, or better still
Fig. 15—A pocket wire gauge which can be purchased from any hardware store handling copper wire.
you can use a regular American Standard Wire Gauge as shown in Figure 15. The slots or holes in which the wire is placed you will notice are numbered to designate the sizes of the wire.
Copper wire sizes number 16 to number 36, 50.82 mils to 5 mils in diameter are generally used in power transformers, radio frequency coils, radio frequency chokes, speakers--in practically every part of a radio receiver or transmitter. The wire is usually round--although sometimes it may be square or rectangular; it may be soft or hard; bare or covered with cotton or silk to insulate it and allow the wires to be placed close together without the current or voltage jumping across; it may be coated with an insulating material
