Imperial units of measurement

The system of imperial units or the imperial system (also known as British Imperial or Exchequer Standards of 1825) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The Imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, although some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.

The Weights and Measures Act of 1824 was initially scheduled to go into effect on 1 May 1825. However, the Weights and Measures Act of 1825 pushed back the date to 1 January 1826. The 1824 Act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.

Apothecaries' units are mentioned neither in the act of 1824 nor 1825. At the time, apothecaries' weights and measures were regulated "in England, Wales, and Berwick-upon-Tweed" by the London College of Physicians, and in Ireland by the Dublin College of Physicians. In Scotland, apothecaries' units were unofficially regulated by the Edinburgh College of Physicians. The three colleges published, at infrequent intervals, pharmacopoeiae, the London and Dublin editions having the force of law.

Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836, the Edinburgh Pharmacopoeia of 1839, and the Dublin Pharmacopoeia of 1850. The Medical Act of 1858 transferred to The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.

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