Over time, however, the metric system gained traction. By the time the American Civil War ended in 1865, most of Europe had adopted the decimal-based measuring system, and the U.S. could no longer ignore it. In 1866, an act of Congress, signed into law by President Andrew Johnson, made it "lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings or court proceedings."
This time when France brought together the world's leading nations nine years later to discuss a new international version of the metric system, the U.S. received an invitation and sent delegates. These nations signed the Treaty of the Meter, establishing the International Bureau of Weights and Measures, an International Committee for Weights and Measures to run the bureau and the General Conference on Weights and Measures to consider and adopt changes. The treaty also specified a lab to be maintained in Sèvres, near Paris, to house the international metric standards, such as the International Prototype Metre, and allowed for these standards to be distributed to each ratifying nation.
The U.S. received its copies of the International Prototype Metre and the International Prototype Kilogram in 1890. The Mendenhall Order of 1893 (named after T.C. Mendenhall, who served as the Superintendent of Weights and Measures at the time) stipulated that the fundamental standards for length and mass in the U.S. be based on metric units. The yard was defined as 3600/3937 meter, and the pound-mass was defined as 0.4535924277 kilogram. In 1959, English-speaking countries agreed on new and improved conversion factors: 1 yard equals 0.9144 meter and 1 pound-mass equals 0.45359237 kilogram exactly.
That means, as of this writing, the U.S. has officially -- and legally -- recognized the metric system for 145 years and has based the units of its standard weights and measures on metric units for almost 120 years. As we'll see on the next page, however, recognition doesn't necessarily translate into practical use.