Research Note

Nautical mile. British, Canadian and American inches

John H Harland

It may come as a surprise to some, that it is just over 40 years since British, American and Canadian feet were standardized. Until that time they differed. In the ordinary way, the difference was so tiny that it could be ignored, but when they got down to very close tolerances, working at a microscopic scale, for instance when designing and building tubes (valves as the Brits called them) where interelectrode distances were critical to the ions moving between the plates. I believe this problem first surfaced in 1939 when Westinghouse were building radar tubes to British design. The specs were in thousandths of an inch .....but the British were talking British inches and the Westinghouse of course were using the American inch, and the tubes from the first production runs did not behave as they should. in the end, NATO settled on the Canadian inch as the standard. See the item below which was posted to Marine History Information Exchange Group .

The article is attributed to Don Hillger, Ph.D, and runs as follows:

"Most people do not realise that the standardisation of the inch for worldwide use did not occur until 1958. Prior to that the inch had been defined differently among the major inch-using countries: the United States, the UK and Canada. Each of these countries had their own definition of the inch, and in each case the inch was defined in terms of metric units, the only set of internationally-accepted standards of length, mass, etc. In the US the metric system was made legal for all purposes, by the Metric Act of 1866, long before any law defined our common US measures. Later, the Mendenhall Order of 1893 defined our common non-metric units in terms of metric units.

"That law regarded metric units as the fundamental and internationally-accepted standards for the United States. It was this law that formally defined the inch based on the conversion factor of 39.37 inches = 1 metre as stated in the Act of 1866. This ratio gives an inch approximately equal to 25.400 05 mm. In Britain the National Physical Laboratory made comparisons of the Imperial Standard Yard to the International Metre, which yielded differing values for the inch over the years. The 1922 value of 25.399 956 mm per inch was arbitrarily selected for use in calibrating the most precise measuring devices. The Canadian Parliament in 1951 established their inch based on a legal definition of the yard as 0.9144 m. This ratio defined the inch as 25.4 mm, a third definition of the inch. This inch was about 2 parts in 106 smaller than the US standard and about 2 parts in 106 larger than the British standard. The differences in definitions of the inch were enough to cause confusion, inefficiencies and difficulties during World War II in attempts to interchange various precision products. It was not until later, in 1958, that the definition of the inch was standardised worldwide as 25.4 millimetres exactly. But that agreement has not completely solved all the problems caused by differing values for the inch. A problem still exists for the foot, where the international foot (based on the 25.4 mm inch) and the survey foot (based on the 25.40005 inch) are both still in use. The US Coast and Geodetic Survey continues to use the survey foot, whereas the rest of industry uses the 25.4 mm inch. This leaves the US with two definitions of the mile, one based on the international foot and the other based on the survey foot. Although this may not seem like much, it causes the two miles to differ by about 3.2 mm, or in 100 miles to differ by about 32 cm!"

{John H Harland}
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