Fig. 817.—Diagram illustrating square mils. A square mil is a unit of area employed in measuring the areas of cross sections of square or rectangular conductors. It is equal to .000001 square inch. One square mil equals 1.2732 circular mils. The figure shows an area of nine square mils; this is equal to 9 × 1.2732 = 11.4588 circular mils.
EXAMPLE.—A copper ribbon for a field coil measures ⅝ inch by ⅛ inch. What is its area in square mils? What is its area in circular mils?
⅝ = .625 in., or 625 mils; ⅛ = .125 in., or 125 mils.
Area in square mils = 625 × 125 = 78,125.
Area in circular mils={78,125 ÷ .7854 }{or 78.125 × 1.2732} = 99,469.
Mil Foot.—This unit is used as a basis for computing the resistance of any given wire. A mil foot means a volume one mil in diameter and one foot long.
The resistance of a wire of commercially pure copper one mil in diameter and one foot long is taken as a standard in calculating the resistance of wires, and has been found to be equal to 10.79 ohms at 75° Fahr.
The calculation is made according to the following rule:
The resistance of a copper wire is equal to its length in feet, multiplied by the resistance of one mil foot (10.79 ohms) and divided by the number of circular mils, or the square of its diameter.
Expressed as a formula:
resistance in ohms = length of wire in ft. × 10.79 circular mils . . . . (1)
EXAMPLE. What is the resistance of a copper wire 1,500 feet long and having a transverse area of 10,381 circular mils?
Substituting these values in formula (1)
resistance= 1,500 × 10.79 10,381 =1.559 ohms.
The transverse area of a copper wire is found by multiplying the resistance of a mil foot (10.79) by its length in feet and dividing the result by its resistance in ohms.