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The rod, perch, or pole (sometimes also lug) is a surveyor's tool [1] and unit of length of various historical definitions. In British imperial and US customary units, it is defined as 16 + 1 ⁄ 2 feet, equal to exactly 1 ⁄ 320 of a mile, or 5 + 1 ⁄ 2 yards (a quarter of a surveyor's chain), and is exactly 5.0292 meters.
Keith Critchlow suggested this may have shrunk 0.63 inches (1.6 cm) from its original length of one megalithic yard over a period of 3000 years. [ 11 ] Thom made a comparison of his megalithic yard with the Spanish vara , the pre-metric measurement of Iberia, whose length was 2.7425 feet (0.8359 m).
Fourteen such rods, including one double cubit rod, were described and compared by Lepsius in 1865. [7] These cubit rods range from 523.5 to 529.2 mm (20 + 5 ⁄ 8 to 20 + 27 ⁄ 32 in) in length and are divided into seven palms; each palm is divided into four fingers, and the fingers are further subdivided. [8] [7] [9]
The 66-foot (20.1 m) chain is divided into 100 links, usually marked off into groups of 10 by brass rings or tags which simplify intermediate measurement. Each link is thus 7.92 inches (201 mm) long. A quarter chain, or 25 links, measures 16 feet 6 inches (5.03 m) and thus measures a rod (or pole).
Flinders Petrie reported on a rod that shows a length of 520.5 mm, a few millimetres less than the Egyptian cubit. [8] A slate measuring rod was also found, divided into fractions of a Royal Cubit and dating to the time of Akhenaten. [9] Further cubit rods have been found in the tombs of officials.
In the classical Akkadian Empire system instituted in about 2250 BC during the reign of Naram-Sin, the finger was one-thirtieth of a cubit length.The cubit was equivalent to approximately 497 mm, so the finger was equal to about 17 mm. Basic length was used in architecture and field division.
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Originally in 1945, the divisions were based on the ring inside diameter in steps of 1 ⁄ 64 inch (0.40 mm). [6] However, in 1987 BSI updated the standard to the metric system so that one alphabetical size division equals 1.25 mm of circumferential length. For a baseline, ring size C has a circumference of 40 mm. [7]