The 1893 Photocopy: How Two Platinum Bars Made America's Inch a Metric Copy

1. The Two Boxes: January 2, 1890, at the White House

The story starts with the Treaty of the Metre, signed in Paris on May 20, 1875, by seventeen nations — the United States among them. The treaty created the International Bureau of Weights and Measures (BIPM) at Sèvres and authorized the construction of prototype meters and kilograms in platinum-iridium: one set to be kept as the international standards, and copies to be distributed to each signatory country. The US copies, cast and calibrated at Sèvres, were shipped across the Atlantic and delivered to the Coast and Geodetic Survey, which had the foresight to have its delegate, George Davidson, escort them under seal.

On January 2, 1890, the packing cases were opened at the White House. President Benjamin Harrison formally accepted Meter No. 27 and Kilogram No. 20, certifying that they arrived in good condition. Two more prototypes arrived the following July and were deposited in the Office of Weights and Measures alongside the first pair. At that moment, the United States possessed physical artifacts that were — by international agreement — copies of the world's best standards of length and mass. The question was what to do with them. For three years, the answer was: nothing. The country kept measuring with the old standards, because changing the legal basis of measurement is not a matter of unpacking a box.

2. The Broken Yard: Why the Old Standards Could Not Hold

Before 1893, the United States defined its length and mass by two aging artifacts inherited from the British system: a brass yard bar and a troy pound kept by the US Mint. The brass bar was a copy of the British Imperial Yard standard — which had itself been destroyed in the 1834 fire at the Houses of Parliament and replaced by a new bar in 1855. The replacement was imperfect: over decades, the Imperial Yard bar was found to be shortening by measurable amounts. The US brass copy was drifting from its nominal length too, and the troy pound was deteriorating.

For a country whose surveyors were mapping a continent and whose industries were measuring in thousandths of an inch, drifting standards were a real problem — a survey in Ohio and a survey in California might disagree not because anyone measured badly, but because the physical standards themselves were changing. Thomas C. Mendenhall, a physicist who had been appointed superintendent of the Coast and Geodetic Survey in 1889, later wrote that the transition to metric-based definitions was "practically forced" on his office by the unreliability of the existing customary standards. The metric prototypes in his custody were demonstrably more stable. He had better artifacts than the law recognized — and he was the person with the authority to change the law's basis.

3. The Order: April 5, 1893, and the 3600/3937 Decision

On April 5, 1893, Mendenhall — in his capacity as Superintendent of Weights and Measures, with the approval of the Secretary of the Treasury — issued the order titled "Fundamental Standards of Length and Mass," published as Bulletin No. 26 of the Coast and Geodetic Survey. It declared that the international meter and kilogram prototypes would henceforth be regarded as the fundamental standards of length and mass in the United States, for both the metric and the customary systems. The yard was defined as exactly 3600/3937 of a meter; the avoirdupois pound as exactly 0.4535924277 kilogram.

The choice of 3600/3937 was deliberate. Mendenhall did not invent the ratio — it came from the Metric Act of 1866, which had made the metric system legal in the United States and set conversion ratios between the systems. And it was chosen for continuity: 3600/3937 ≈ 0.9144018 m was the value that kept the new legal yard essentially equal to the old brass yard, so that ongoing survey work, industrial tooling, and land records were not disrupted by an arbitrary jump. The order was an administrative act, not a law — Mendenhall needed no new legislation because the Metric Act of 1866 had already legalized the metric system and its ratios. What the order did was make those ratios the official, binding basis of all American measurement, replacing the unreliable artifacts with the platinum prototypes. The retooling guide tells what happened when those redefined units met America's machines; this article is the story of the standards themselves.

4. The Photocopy: What It Meant to Redefine the Inch

The quiet radicalism of the Mendenhall Order is easy to miss, because nothing changed in daily life. A foot was still a foot; a pound was still a pound; rulers still said "inches." What changed was the legal definition underneath. Before 1893, the inch was an independent unit defined by a brass bar. After 1893, the inch was derived — defined as one 36th of a yard, where the yard was 3600/3937 of a meter. The customary units became photocopies of the metric system, made to look like the originals.

This is the inversion that surprises everyone who hears it: America's units are not a separate system that happens to coexist with metric. They are metric fractions wearing customary names, and they have been since 1893. The yard's 3600/3937 fraction made the inch equal to 1/39.37 of a meter — about 0.02540005 m, a hair longer than the modern 2.54 cm. The difference was a few parts per million: invisible to a carpenter, critical to a gage maker, and exactly the kind of drift the order was designed to eliminate. The length guide documents the constants that came out of this; the point here is the mechanism — an administrator, two platinum bars, and the legal authority to say which standard is the original and which is the copy.

5. The 1959 Refinement: From 3600/3937 to 0.9144

For sixty-six years, the 3600/3937 yard and the 0.4535924277 kg pound were the law. Then, in 1959, the English-speaking nations — the United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa — signed the international yard-and-pound agreement, which replaced the awkward fractions with clean exact values: 1 yard = 0.9144 m exactly, and 1 pound = 0.45359237 kg exactly. The change was tiny — about two parts per million on the yard, a few parts in ten million on the pound — but it was a genuine redefinition, and it mattered enormously to the people whose business was precision.

The 1959 agreement is why the constants on this site are what they are. One inch = 2.54 cm exactly, because 0.9144 m ÷ 36 = 0.0254 m. One foot = 0.3048 m exactly, because 0.9144 ÷ 3 = 0.3048. The inches to cm converter, the feet to meters converter, and the lbs to kg converter all quote 1959 treaty law. And the change was not free: every gage block, every reference standard, and every calibration chain in the country had to be re-derived from the new values — a nationwide recalibration measured in parts per million, the story the retooling guide tells from the machine side. The weight & mass guide carries the pound side of the same agreement.

6. The Inheritance: Law, Gage Blocks, and Every Converter on This Site

The chain from Sèvres to your ruler has four links: the 1875 treaty created the international prototypes; the 1890 White House acceptance put copies in US custody; the 1893 order made them the legal basis of the yard and pound; and the 1959 agreement cleaned the values to exact decimals. The physical prototypes themselves have since retired from duty — the meter was redefined by the speed of light in 1983, and the kilogram by Planck's constant in 2019 — but their legal legacy is untouched. The inch is still a metric fraction. The pound is still a metric fraction.

That legacy is why this article ends where this site begins. Every converter on EnginStack that shows an "exact" constant is showing a legal definition: 1 in = 2.54 cm is law, not approximation; 1 lb = 0.45359237 kg is law; 1 N·m = 0.7375621493 ft·lb is the 1959 pound and the 0.3048 foot combined. The photocopy taken in 1893 is still in force, refined once in 1959 and never revoked. The full context — why a country whose standards are metric still measures its life in inches — is the history hub this article expands; the machine-level mechanism of why retooling never happened is the retooling deep dive. The standards were the easy part. The machines were not.

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