Yard

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A yard (abbreviation: yd) is a unit of length in several different systems, including English units, Imperial units, and United States customary units. It is equal to 3 feet or 36 inches, although its length in SI units varied slightly from system to system. The most commonly used yard today is the international yard, which is defined to be exactly 0.9144 metre.

Contents

Equivalence to other units of length

1 international yard is equal to:

  • 3 feet (1 foot is a third of a yard)
  • 36 inches
  • 0.9144 metre (1 metre is equal to about 1.0936 international yards)[3]

The early yard was divided by the binary method into two, four, eight, and sixteen parts called the half-yard, span (unit), finger (unit), and nail (unit). Two yards are a fathom.

Historical origin

The yard derives its name from the word for a straight branch or rod,[4] although the precise origin of the measure is not definitely known. Some believe it derived from the double cubit, or that it originated from cubic measure, others from its near equivalents, like the length of a stride or pace. One postulate was that the yard was derived from the girth of a person's waist, while another claim held that the measure was invented by Henry I of England as being the distance between the tip of his nose and the end of his thumb.[5] It was first defined in law by Edward I of England in 1305,[6][7][8] and again by Edward III of England in 1353.[9]

Following Royal Society investigations by John Playfair, Hyde Wollaston and John Warner in 1814 a committee of parliament proposed defining the standard yard based upon the length of a seconds pendulum. This was enacted in 1824.[10] This is 39.1392 inches, and can be derived from the number of beats (86,400) between two meridians of the sun. The temperature compensated pendulum was to be held in a vacuum at sea level in Greenwich, London to give the length of the standard yard. However, a new physical Imperial Standard Yard was authorised by the Weights and Measures Act 1878,[11] and was the legal standard in the United Kingdom until 1964. The "United Kingdom primary standard of the yard" (the old "Imperial Standard Yard") was measured as 0.914 396 9 metres in May 1963, one part in 300,000 shorter than the international yard that became the new legal standard, and was found to be shortening at a rate of about one part per million every 23 years.[12]

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