Imperial and US customary measurement systems

The furlong and the acre, once it became a measure of the size of a piece of land rather than its value, remained relatively unchanged.

Later in the century, efforts were made to align the definition of the pound and the yard in the two countries by using copies of the standards adopted by the British Parliament in 1855.

[2] The earliest records of English units of measure involve the weight (and therefore the value) of Saxon coins.

[3]: 122–123 [11] During the medieval era, agricultural products other than wool were mostly sold by volume, with various bushels and gallons being introduced over the years for different commodities.

In the early fourteenth century the wool trade traditionally used the avoirdupois system of weights, a process that was formalized by Edward III in 1340.

In 1496 Henry VII ordered that reference copies of the yard, pound and gallon should be made of brass and distributed to specified towns and cities throughout the kingdom.

[12]: 90–91  The Acts of Union 1800 which united Ireland with Great Britain had less of an effect on weights and measures—Irish weights and measures having been based on the English foot and pound avoirdupois since 1351, though the Irish acre and mile were based on a perch of 7 yards, not 5+1⁄2 yards as in England.

Even though not explicitly permitted by statute, many markets used bushels based on weight rather than volume when selling wheat and barley.

The standard imperial yard was not stable – in 1947 its rate of shrinkage was quantified and found to be one part per million every 23 years.

[3]: 155  In 1960 representatives from the NPL and other national laboratories from the United States and Commonwealth agreed to redefine the yard as being exactly 0.9144 metres, an action that was ratified by the British Government as part of the Weights and Measures Act 1963.

Initially this metrication was voluntary and by 1985 many traditional and imperial units of measure had been voluntarily removed from use in the retail trade.

The Weights and Measures Act 1985 formalized their removal for use in trade, though imperial units were retained for use on road signs and the most common imperial weights such as the foot, inch, pound, ounce, gallon and pint continued to be used in the retail trade for the sale of loose goods or goods measured or weighed in front of the customer.

[30] The Articles of Confederation, which predated the Constitution, gave the central government "the sole and exclusive right and power of...fixing the Standard of Weights and Measures throughout the United States.

[32] During the First Congress of the United States in 1789, Thomas Jefferson was detailed to draw up a plan for the currency and weights of measures that would be used in the new republic.

His report suggested a means of manufacturing a local standard and also left the way open for an adoption of a decimal-based system should this be appropriate.

Congress did nothing, but Ferdinand Hassler, Superintendent of the East Coast survey, who in 1790 had met using contacts in his native Switzerland acquired a copy of the [French] mètre des Archives.

[34] In 1821 John Quincy Adams, then Secretary of State submitted a report based on research commissioned by the Senate in 1817 which recommended against adoption of the metric system.

A meeting was held the following year and subsequently became an annual gathering known as the National Conference on Weights and Measures (NCWM).

[40] Inactivity by Congress and the lack of uniformity of weights and measures which were crippling US economic growth in the nineteenth century led to the National Bureau of Standards to call a meeting of states in 1905 which resulted in the setting up of the National Conference on Weights and Measures (NCWM).

[43] During the twentieth century the principal change in the customary system of weights and measures was an agreement between NIST and the corresponding bodies in Australia, Canada, New Zealand, South Africa and the United Kingdom, signed in 1960, that redefined the yard and the pound in terms of the metre and the kilogram respectively.

The British thermal unit (Btu) is defined as the heat needed to raise the temperature of one pound of water by one degree Fahrenheit.

Both systems depend on the gravitational acceleration, and use the pound-force as the unit of force but use different approaches when applying Newton's laws of motion.

[54] On the other hand, the EE system uses a different approach and introduces the acceleration due to gravity (g) into its equations.

Both these approaches led to slight variations in the meaning of the pound-force (and also of the kilogram-force) in different parts of the world.

[58] The standard yard and [Troy] pound were lost in 1834 when a fire partially destroyed the Palace of Westminster.

By the end of the Second World War, the standards laboratories of Canada, Australia, New Zealand and South Africa also had their own copies of the pound and the yard.

These legal and technical discrepancies, described by McGreevy (pg 290) as being "unsound" led to the Commonwealth Science Conference of 1946.

[15][64] proposing that the Commonwealth countries and the United States should all redefine the yard and the pound in terms of an agreed fraction of the metre and kilogram respectively.

However the introduction of the metric-based Ordnance Survey National Grid in the United Kingdom in 1938 meant that British surveyors were unaffected by the change.

Apart from the poundal, most of the named units of measure are non-coherent, but were adopted due to traditional working practice.