The imperial unit system, also known as British unit system was established by the Weights and Measures Act (WMA, 1824) fo June 17th, 1824. Its three base units are the pound, the yard, and the second. The yard was defined in 1878 as the distance at 62 [°F] (20 [°C] or 293 [K]) between a pair of lines etched in gold plugs set in a bronze bar. Earlier in 1856, the pound was defined in terms of the mass of a platinum cylinder, known as the Imperial standard Pound, and both were kept in the Standards Department of the Board of Trade in London. The imperial unit of capacity was the gallon, which was defined as the volume of 10 pounds of distilled water, weighed in air against brass weights at a temperature of 62 [°F] (16.67 [°C], or 289.8167 [K]) and atmospheric pressure of 30 inches of mercury. These leagal measures were then used in all countries of the British Commonwealth. To a close approximation, the metric equivalents of these imperail pound and yard standards are 0.45359243 [kg] and 0.91443992 [m], respectively.
The WMA in 1963 modified the nature of these standards, and redefined the units in terms of the kilogram and metre standards maintained in Paris. These new precise definitions of the pund and the yard are 0.45359237 [kg] and 0.9144 [m], respectively. It should be noted that the unit of time, the second, was always the same in both systems and was redefined by the 13th CGPM in 1968.
In 1980, Parliament approved a Statutory Instrument which began the progressive phasing out of the imperial system by withdrawing authorization of a substatial number of units, such as the British thermal unit (Btu), the cran, the furlong, the horsepower (HP), the hundredweight (cwt), the ton and the Fahrenheit degree (°F). In 1985, the curie, rem, and rad were discarded in favour of the bacquerel, sievert, and gray repsectively, although the legal status of the pound and yard was reaffirmed by the WMA of 1985.
From November 1995, the United Kingdom officially adopted the metric units for general use and business transactions. As a result, the metre and kilogram are now compulsory in place of the yard nad pound. Currently legislation has decreed that from October 1st, 1995 for economic, public health, public safety and administrative purposes, only metric units are allowed except for the following, which may be used unitl December 31st, 1999:
The WMA in 1963 modified the nature of these standards, and redefined the units in terms of the kilogram and metre standards maintained in Paris. These new precise definitions of the pund and the yard are 0.45359237 [kg] and 0.9144 [m], respectively. It should be noted that the unit of time, the second, was always the same in both systems and was redefined by the 13th CGPM in 1968.
In 1980, Parliament approved a Statutory Instrument which began the progressive phasing out of the imperial system by withdrawing authorization of a substatial number of units, such as the British thermal unit (Btu), the cran, the furlong, the horsepower (HP), the hundredweight (cwt), the ton and the Fahrenheit degree (°F). In 1985, the curie, rem, and rad were discarded in favour of the bacquerel, sievert, and gray repsectively, although the legal status of the pound and yard was reaffirmed by the WMA of 1985.
From November 1995, the United Kingdom officially adopted the metric units for general use and business transactions. As a result, the metre and kilogram are now compulsory in place of the yard nad pound. Currently legislation has decreed that from October 1st, 1995 for economic, public health, public safety and administrative purposes, only metric units are allowed except for the following, which may be used unitl December 31st, 1999:
- pounds and ounces for weighing of goods sold in bulk
- pints and fluid ounces for alcoholic beverages such as beer and cider, waters, lemonades, and fruit juices in returnable containers
- therms for gas supply;
- Fathoms and nautical miles for marine navigation.
- statute miles, yards, feet and inches for road traffic signs and related measurements of speed and distance
- pints for dispensing draught beer and cider, and for milk in returnable containers
- acres for land registration purposes and surveyors measurements
- troy ounces for transactions in gold and other precious metals
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