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.
Metric equivalents in this article usually assume the latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 398415 metres. 0.914
In 1824, the various different gallons in use in the British Empire were replaced by the imperial gallon, a unit close in volume to the ale gallon. It was originally defined as the volume of 10 pounds (4.5 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C). In 1963, the gallon was redefined as the volume of 10 pounds of distilled water of density 859 g/mL weighed in air of density 0.998217 g/mL against weights of density 0.001, which works out to 8.136 g/mL096 l or 4.546. The Weights and Measures Act of 1985 switched to a gallon of exactly 277.4198 cu in09 L (approximately 4.546). 277.4194 cu in
These measurements were in use from 1826, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971. In the USA, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.
In the 19th and 20th centuries, the UK used three different systems for mass and weight: