Thomas C. Mendenhall, 18 May 1918. Image source: T.C.Mendenhall-1918.jpg by Hell&Fischer Philadelphia is licensed under Public Domain Mark 1.0
In 1893, Thomas C. Mendenhall (1841–1924), a native of Hanoverton, Ohio, decided that the international meter and kilogram would be the fundamental length and mass standards for weights and measures in the United States. Mendenhall was then Superintendent of Weights and Measures for the U.S. federal government. His decision, known as "The Mendenhall Order," was a major departure from past United States policy of maintaining length and mass standards identical to the Imperial systems of weights and measures used in Great Britain. In that system, length was the English yard and mass the pound.
Mendenhall (1841-1924) was a renowned scientist, appointed by President James Harrison as superintendent of the U.S. Coast and Geodetic Survey in 1889. He had been chairman of physics at Ohio State University, chief of the Instrument Division in the U. S. Signal Corps, and president of Rose Polytechnic Institute.
The Mendenhall Order redefined customary British-U.S. units in terms of metric units. The yard became 3600/3937 meter and the avoirdupois pound-became the mass of 0.4535924277 kilogram. The National Bureau of Standards (now the National Institute of Standards and Technology) used those same definitions from its founding in 1901 until 1959. In 1959, English-speaking countries agreed to define one yard as 0.9144 meter and one pound-mass as 0.45359237 kilogram.
NIST LC 1140 – The Metric Pyramid is a helpful study aid that can be easily constructed with yellow cardstock to keep common approximate unit conversion factors for mass, length, area, volume, temperature, and energy close at hand. Image source: NIST LC 1140 Metric Pyramid by The National Institute of Standards and Technology (NIST), agency of the U.S. Department of Commerce is licensed under Public Domain Mark 1.0
Mendenhall did not need any new laws for the action because the metric system had been legal in the U. S. since passage of the Metric Act of 1866. It regarded metric units as the fundamental and internationally accepted standards for the United States.
President Harrison appointed Mendenhall director of the U.S. Coast and Geodetic Survey in1889, and he served until 1894. The weights and measures office then was part of the geodetic survey. Mendenhall helped to determine the boundary line between the United States and Canada, including the boundary of Alaska. As a member of the International Electrical Congress, he also had a hand in defining basic units of electricity.
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Scale of Things Chart. Image source: Basic Energy Sciences, U.S. Department of Energy, U.S. DOE Version 01-18-05, pmd by Office of Science, U.S. Department of Energy is licensed under Public Domain Mark 1.0