pound avoirdupois

A unit of mass = 453.592 37 grams (now, technically, the international pound), now used chiefly in the United States, but formerly the most commonly encountered unit of mass throughout the English-speaking world. The magnitude of the pound avoirdupois has varied less than 1% since the middle of the 14th century.

History

What does “avoirdupois” mean?

This term originally referred to a class of merchandise: aveir de peis, “goods of weight,” things that were sold in bulk and were weighed on large steelyards or balances. Only later did it become identified with a particular system of units used to weigh such merchandise. The imaginative orthography of the day and the passage of the term through a series of languages (Latin, Anglo-French and English) has left many variants of the term, such as haberty-poie and haber de peyse. (The Norman “peis” became the Parisian “pois”. In the 1600's “de” was replaced with a hypercorrect “du”, provoking the disdain of the writer of the entry in the Oxford English Dictionary).

Origin

The pound avoirdupois seems to have appeared in the late 13th to middle 14th century. During and slightly after this period several pounds were in use:

In the competition between these pounds the pound avoirdupois won out. (Another pound, the Tower pound, was used only in the operations of the Mint, not in commerce.)

Why the pound avoirdupois replaced the liber mercatoria

The first reason is 15 ounces to the pound. The mere fact that 15 is not an even number counts against it. That it has only two integral divisors, 3 and 5, is still worse. (The troy pound has 2, 3, 4 and 6!) Sixteen, on the other hand, can be cut in half, in quarters and in eighths and the result is still a whole number of ounces. In this decimal age, it is hard for us to appreciate how powerfully useful a feature that is. It is hard to find a fifteenth of a pie with a knife, but it is easy to find a sixteenth.

The second reason concerns the persons who used the unit. The avoirdupois pound arose in the wool trade. The merchants who dealt in wool were international traders; wool and woolen cloth was to the 14th century what oil was to the 20th: the single largest item in international commerce. Trade with its largest customer, Italy was facilitated by a common unit of mass, namely the avoirdupois ounce.

Once the balance in the marketplace had avoirdupois weights for weighing wool, it was convenient to use the same weights to weigh other commodities. The avoirdupois pound was in use for a long time before it ever appeared in law. The City of Winchester has a full set of avoirdupois standards from the reign of Edward III, probably made in 1357.

Recovery under Elizabeth I

In 1558, the first year of her reign, Elizabeth issued a new set of standards. They were badly made and the public became convinced they were heavier than the standards that had been in use. Weighings in 1873 of surviving weights from this series indicated the public was right.

In 1574, a new jury of 9 merchants and 12 goldsmiths impaneled with the goal of restoring the “ancient” standards. The 1558 weights were recalled, and a new set of weights issued. The troy weights were based on those at the Goldsmiths’ Company in London, which the jury compared with the troy standards at the Mint. Unfortunately, the jurors for this second series made two fundamental mistakes: they thought that the avoirdupois pound consisted of 15 troy ounces and that the troy pennyweight was the weight of a penny. Further detracting from their usefulness, the weights were badly made. Much grumbling ensued.

In 1582, the weights issued in 1574 were recalled and a new jury impaneled, consisting of 18 merchants and 11 goldsmiths. The Goldsmiths’ troy standards were again taken as the basis for troy weight, but the avoirdupois pound was based on a 56-pound weight from the time of Edward III, found in the Exchequer. This third series, issued in 1588, was very well-made in bronze, and the prototype of the pound deposited in the Exchequer remained the national standard until 1824. It can be seen today in the Science Museum in London.

The pound avoirdupois in the 19th and 20th centuries

In 1827, Albert Gallatin, U.S. minister at London, procured a brass troy weight that was a copy of the British imperial troy pound. On Oct 12, 1827, the weight was ceremoniously unpacked at the Mint in Philadelphia by President Adams. Congress then adopted this weight on May 19, 1828, as “the standard troy pound of the Mint of the United States, conformably to which the coinage thereof shall be regulated.”

In fact, by default this standard troy pound became the basis, not just of the troy pound, but of all U. S. weights. The pound avoirdupois, for example, equaled ⁷⁰⁰⁰/₅₇₆₀ of the troy pound.

In May 1830, Congress passed a bill directing the Treasury Department to compare the standards of weights and measures in use at the various customs houses. Some legislators feared that their ports were being shortchanged. A port using an overweight standard pound would enjoy an advantage, because goods passing through it would pay lower tariffs. The Treasury’s report (1832) confirmed the fears of the bill's proponents: the standards varied greatly, and the Treasury immediately started replacing the various customhouse standards. In this process, which had not been called for by legislation, many of the weights and measures of the United States were defined for the first time. In 1836, Congress directed Ferdinand Hassler to make and deliver to each of the states a set of standards. On July 7, 1838, it directed him to give each state a balance. The last such set of standards was delivered to the State of Hawaii in 1960. Thus a uniform pound avoirdupois spread thrpought the nation.

On July 28, 1866 Congress passed an act making use of the metric system lawful in the United States, and in this law the kilogram is defined as 2.2046 pounds.

A comparison of the British Imperial Standard Pound and the International Prototype Kilogram made in 1883 led to the equivalence 1 imperial pound = 0.453 592 427 7 kilogram, which both Britain and the United States adopted.

In 1889 Great Britain rounded the result off, officially adopting the definition 1 Imperial Pound = 0.453 592 43 kilogram. As a result, from that date until 1959, the U.S. and British pounds differed.

A new standard, Kilogram No. 20 from the International Bureau of Weights and Measures, was unpacked at the White House in the presence of President Harrison on January 2, 1890. While in Britain the pound was based on a British prototype, starting in 1893 (the Mendenhall order) the Americans defined their pound in terms of the International Prototype of the Kilogram, using the equivalence arrived at by the comparison of 1883. This decision was reconfirmed when the National Bureau of Standards was established in July, 1901. From 1901 to July 1, 1959, the U.S. pound avoirdupois = 0.453 592 427 7 kilogram.

The International Pound

Effective July 1, 1959, by mutual agreement a new International Pound = 0.453 592 37 kilogram became effective for scientific and technical work in Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States. In the United States, the new value was accepted as the value of the pound avoirdupois for all purposes, as it was in the United Kingdom by the Weights and Measures Act of 1963, in which, for the first time, the British government defined the pound in terms of the kilogram. The new International Pound is about 1 part in 10 million smaller than the U. S. pound avoirdupois that it replaced.

Changing Values of the Pound Avoirdupois

pound avoirdupois	1588 – 1824	exactly the mass of the prototype in the Exchequer, about grams
imperial pound	1824-1959	exactly the mass of the prototype, approximately 0.453 592 338 kilogram
U.S. pound avoirdupois	18 – 1901	exactly 7000/5760 of the mass of the “troy pound of the Mint”
U.S. pound avoirdupois	1901 – 1959	exactly 0.453 592 427 7 kilogram
international pound	1959 – present	exactly 0.453 592 37 kg

resources

Ronald Edward Zupko.
British Weights and Measures. A History from Antiquity to the Seventeenth Century.
Madison, WI: Univ. of Wisconsin Press, 1977.

U.S. Dept. of Commerce. National Bureau of Standards.
Louis V. Judson.
Weights and Measures Standards of the United States. A Brief History.
Miscellaneous Publication 247.
Washington, D.C.: U. S. Government Printing Office, October 1963.