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MANAGERS AND PROJECTIONISTS 77
individual projectionist and exhibitor to figure out his own problem.
The data supplied is important, because by its intelligent application it will very often be found that much money is wasted in the excessive resistance of circuits, which could be avoided by installing wires of larger size. It is therefore, we repeat, essential that the projectionist and manager have a good working knowledge of matters of this sort.
THE MEASUREMENT OF WIRES.— The area of crosssection of electrical conductors of various kinds is measured either in square or circular mils, the latter being used for round wires and the former for square or rectangular conductors.
A circle measuring 1/1000 of an inch in diameter is called a "circular mil," the same being commonly abbreviated "CM."
A round wire 1/1000 of an inch in diameter is said to have an area of cross-section of one circular mil, "cross-section" being the surface of the end of a wire.
The area of round wires is directly proportioned to the square of their diameters, the calculation being made in circular mils.
"Squaring the diameter" means multiplying the diameter by itself, thus : if a wire be 10 mils in diameter, then 10 x 10 = 100 is the square of its diameter, hence the area of its cross-section in CM.
Let us consider a wire having a diameter of Y^ of an inch. Since one inch is equal to one thousand thousandths (1000/ 1000), the diameter of the wire expressed in thousandths of an inch, or mils, would be equal to 1000 -4 4 = 250. A wire % of an inch in diameter is, expressed after the electrical fashion, 250 mils in diameter, and since the area of cross-section of a wire in circular mils is equal to the square of its diameter in mils, it follows that our quarter-inch-diameter wire would have an area of 250 x 250 = 62,500 circular mils.
The circular mill area of any round wire may be found by measuring its diameter in thousandths of an inch, using a micrometer caliper or wire gauge for the purpose and multiplying the measurement thus obtained by itself. The result will be the CM. area of the wire.
The capacity of any round wire may be found by measuring the wire diameter as above set forth, multiplying the measurement by itself and comparing the result with "Area in CM." column in wire capacity table, page 70.