This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
For designating the diameters of wire, thicknesses of sheet metals, and thicknesses of the walls of tubes, various arbitrarily chosen scales of sizes are used in America and in Europe. These sizes are designated as wire-gage units. For example, a wire may be designated as No. 5, B. & S. (Brown & Sharpe) or No. 5, B. W. G. (Birmingham Wire Gage).
The several systems use numbers to designate sizes ranging from 0000000 (usually expressed as 7/0) to 50, although the sizes are of different dimensions in the different systems.*
In America the B. & S. gage (prepared by Messrs. Brown & Sharpe Mfg. Co., Providence, B. I.) is standard among manufacturers for designating wire sizes and metal thicknesses, although the U. S. Navy Department has adopted the B. W. G. (Birmingham
* A table in the Appendix gives a comparison of the different wire-gage systems.
Wire Gage) for designating thickness of walls of pipes and tubes, and the U. S. standard gage for designating steel and iron-plate thicknesses.
The finest wire drawn is a copper wire of about .001-inch diameter.