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Electricity and Electronics for Renewable Energy Technology by Ahmad Hemami

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Appendix D: Mil and Circular Mil

In the English system for measuring small lengths such as paper thickness and precision machining dimensions 1/1000 of an inch is used as the unit. It is called mil. For items that are round and have a circular cross section, like most wires, another small unit is used. This unit, called circular mil, is a unit of measure for area. One circular mil is the area of a circle whose diameter is 1 mil. In this respect, the area called a circular mil is

1 CM=14(1)2(π)=π4×106 in2=π4×0.00064516 mm2=0.00050671 mm2

If the diameter of a wire is d mils, then its area is

14(d)2(π)=d2CM

By defining the circular mil, in fact, the multiplication by the factor π/4 is included in the unit, making the expression for measurement ...

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