In England and the United States, at least as early as the 18^{th} – 21^{st} century, now a quantity of roofing material, such as thatching¹, shingles or slates, sufficient to cover 100 square feet of roof.

It was originally also applied to covering other surfaces, for example in tiling a floor.²

Alexander (1850).

Page 134: “Architects and Builders reckon 1 square = 100 square feet.”

1. James Britten.

*Old Country and Farming Words.*

English Dialect Society, number 30.

London: Trübner and Co., 1880.

Page 176.

2. John Dean.

*The rule of practice methodized and improved. Wherein are contained all the
... And to measuring by the square of 100 feet, applied to flooring, tyling, and
partitioning....*

London: printed for G. Keith, 1756.

Rare. Harvard has a copy; the bookseller John Drury offered another (catalog 109, #18).

A unit-like term used in expressing the electrical resistance of conducting materials used as very thin flat sheets, for example the indium-tin-oxide coating on LCD displays, or the layers that make up integrated circuits. Although at one time experts¹ proposed describing sheet resistance simply in “squares,” it is now only encountered as part of the phrase “ohms per square” (symbol, Ω/□). In actual practice, sheet resistance is measured with a four-point probe. Conceptually, however, think of it as being measured by feeding a current into the entire length of one edge of a square sheet and taking it out along the entire length of the opposite edge.

A bigger square will be wider (and so more conductive), but being a square it will also be longer (and so have higher resistance, because the contacts are farther apart). These effects cancel each other out, so the sheet resistance of a square sheet of any particular material is the same no matter what the linear dimension of the square's edge. In other words, you get the same answer whether it's measured in ohms per square inch, per square micrometer, or per square mile. Since the square's actual size is irrelevant, it is omitted - it's just ohms per square.

1. R. W. Berry, P. M. Hall and H. T. Harris.

*Thin Film Technology.*

New York: Van Nostrand Reinhold Company, 1968.

Math showing how a four-point probe works: www.lxdinc.com/AppNotes/Ohms_per_Square.htm

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Last revised: 17 May 2006.