A circular measure was used in comparing circular cross-sections, e.g., of wires, etc. A circular unit of the ares is the area of the circle whose diameter is one linear unit.[1]

For example, 1 circular mil is equivalent to 0.7854 square mil in area, 1 circular millimeter = 1550 circular mils = 0.7854 square millimeter. Here [1]

References

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  1. ^ a b "Civil Engineer's Pocket-Book" p. 889, Wiley, 1904