History
The fluid ounce was originally the volume occupied by one ounce of some substance such as wine (in England) or water (in Scotland). The ounce in question varied depending on the system of fluid measure, such as that used for wine versus ale. Various ounces were used over the centuries, including the Tower ounce, troy ounce, ounce avoirdupois, and various ounces used in international trade such as Paris troy. The situation is further complicated by the medieval practice of "allowances" whereby a unit of measure was not necessarily equal to the sum of its parts. For example, the 364-pound woolsack had a 14-pound allowance for the weight of the sack and other packaging materials.
In 1824, the British Parliament defined the imperial gallon as the volume of ten avoirdupois pounds of water. The gallon was divided into four quarts or eight pints. Each pint was divided into four gills or twenty fluid ounces, that is, five ounces for each gill. Thus, the mass of an imperial fluid ounce of water is one avoirdupois ounce (28.4 g). The US fluid ounce is based on the US gallon, which is based on the wine gallon, used in England prior to 1824.
Read more about this topic: Fluid Ounce
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