The Mendenhall Order marked a decision to change the fundamental standards of length and mass of the United States from the customary standards based on those of England

Imperial unit

The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...

 to metric standards. It was issued on April 5, 1893 by Thomas Corwin Mendenhall

Thomas Corwin Mendenhall

Thomas Corwin Mendenhall was an autodidact US physicist and meteorologist.-Biography:Mendenhall was born in Hanoverton, Ohio to Stephen Mendenhall, a farmer and carriage-maker,...

, superintendent of the U.S. Coast and Geodetic Survey, with the approval of the United States Secretary of the Treasury

United States Secretary of the Treasury

The Secretary of the Treasury of the United States is the head of the United States Department of the Treasury, which is concerned with financial and monetary matters, and, until 2003, also with some issues of national security and defense. This position in the Federal Government of the United...

, John Griffin Carlisle

John Griffin Carlisle

John Griffin Carlisle was a prominent American politician in the Democratic Party during the last quarter of the 19th century. He served as the Speaker of the United States House of Representatives from 1883 to 1889 and as United States Secretary of the Treasury from 1893 to 1897 during the Panic...

. The order was issued as the Survey's Bulletin No. 26 - Fundamental Standards of Length and Mass.

Standards before the order

In October 1834 the United Kingdom Houses of Parliament

Palace of Westminster

The Palace of Westminster, also known as the Houses of Parliament or Westminster Palace, is the meeting place of the two houses of the Parliament of the United Kingdom—the House of Lords and the House of Commons...

 were destroyed in a fire

Burning of Parliament

Burning of Parliament is the popular name for the fire which destroyed the Palace of Westminster, the home of the Parliament of the United Kingdom, on 16 October 1834...

, and the British standards of length and mass were also destroyed. "When the new imperial standards to replace them were completed in 1855 two copies of the yard and one copy of the avoirdupois

Avoirdupois

The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...

 pound were presented to the United States" (Barbrow & Judson 1976, 9). These were superior to the yard then in use, so one of them was adopted as the United States national standard yard. These yards were taken to England and re-compared with the imperial yard in 1876 and 1888. The pound provided by the United Kingdom agreed with the United States mint pound, which remained the national standard according to Barbrow and Judson (the NIST virtual museum claims the UK pound replaced the mint pound).

These were the fundamental standards for customary length and mass measurements in the United States, but the Office of Weights and Measures had other standards for metric measurements.

Official recognition of the metric system

In 1866 the Congress passed a law which allowed, but did not require, the use of the metric system. Included in the law was a table of conversion factors between the traditional and metric units. The U.S. Coast and Geodetic Survey Office of Weights and Measures had on hand a number of metric standards, and selected the iron "Committee Meter" and the platinum "Arago Kilogram" to be the national standards for metric measurement; the standard yard and pound previously mentioned continued to be the standards for customary measurements (Barbrow & Judson 1976, 10).

A series of conferences in France between 1870 and 1875 led to the signing of the Metre Convention and to the permanent establishment of the International Bureau of Weights and Measures

International Bureau of Weights and Measures

The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

, abbreviated BIPM after the French name. The BIPM made meter and kilogram standards for all the countries that signed the treaty; the two meters and two kilograms allocated to the United States arrived in 1890, and were adopted as national standards (Barbrow & Judson 1976, 16).

Reasons for the change

The imperial standard yard of 1855 was found to be unstable and shortening by measurable amounts (Barbrow & Judson 1976, 16). Also, the mint pound was found to be "likewise unfit for use." For several years before the Mendenhall order was actually issued, the Office of Weights and Measures was "practically forced" to use the metric standards because of their superior stability, and because they were better designed for carrying out precision comparisons. The Office found that the conversion tables in the 1866 law were satisfactory and used them to derive customary length and mass from the metric standards. The conversions were 1 yard = 3600/3937 meter and 1 pound = kilogram. The Mendenhall order amounted to a formal announcement of a change that had already occurred (Mendenhall 1893).

Refinement of the conversions

The definitions of 1893 remained unchanged for 58 years, but increasing precision in measurements gradually made the differences in the standards in use in English-speaking countries important. Effective July 1, 1959, Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States agreed that 1 yard = 0.9144 meter and that 1 avoirdupois pound = kilogram (but see U.S. survey foot in the foot article) (Barbrow & Judson 1976, 20).

Standards versus systems

Mendenhall ordered that the standards used for the most accurate length and mass comparison change from certain yard and pound objects to certain meter and kilogram objects, but did not require anyone outside of the Office of Weights and Measures to change from the customary units to the metric system.