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.
Welded pipe is commonly seen and much used as steam, gas and water pipe, and is commercially known as iron pipe. It is made in standard sizes designated in inches as follows, viz.: 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, 2, 2 1/2 3, 3 1/2, 4, 4 1/2, and in sizes of even inches from 5 to 12, both inclusive. These sizes refer to the inside diameters of the pipe, but, as a matter of fact they are only nominal, as the actual diameters differ more or less from the designated diameters. While iron pipe is referred to by its nominal inside diameter, it is standardized in size in outside diameters.*
Iron pipe is marketed either galvanized or plain, and some makers coat pipe with asphalt or tar.
For very heavy pressures, or for driving oil or artesian wells, two special thicknesses of iron pipe are made, known commercially as extra strong and double extra strong. These have the same external diameters as the standard piping. Also, iron pipe is made much larger than 12 inches in diameter for many uses.
* Paragraph 437 of the Appendix gives a table of sizes and dimensions of standard iron pipe.
Iron pipe is much used as underground conduits for electric wires.