On Fri 15 Jul 2016 at 22:48:11 (-0500), John Hasler wrote: > Dennis writes: > > BTW: one inch now equals 2.54 cm *exactly*, in case you haven't been > > keeping up! (Used to be approx 2.54 cm.) This is what I mean by > > arbitrary. Don't like the conversion ratio? Then just change it! > > It wasn't a change in conversion ratio. It was change in definition. > Originally US length measures were defined by a physical standard > derived from the British yard such that the inch worked out to close to > 2.54cm. In 1893 the inch was defined as 2.54cm
Close, but no ciger. The 1866-07-28 length of the US yard was preserved at 3600รท3937 of the metre. What changed in 1893 was the adoption of the International Standard Metre (21 and 27 were the ones they received) as the prototype. The change to inch=2.54cm came much later (1959-07-01) and was actually expressed as one yard = 0.9144 of the metre. The old foot, the US Survey foot, is still used by the National Geodetic Survey for the publication of heights. Some states still publish their State Plane Coordinates in feet (provided to them by NGS in metres) and are at liberty to use either type of foot. > and BIPM-supplied meter > and kilogram standards became the official US measurement standards. It > took until 1930 for the Brits to catch up. Who was first is of little real interest. What's difficult to argue against is that the Brits have pretty well gone metric with a few exceptions like distances between towns, beer glasses, and the freedom to sell milk in pints or litres. The Americans have not. Even scientists convert their statements into customary units when they commnicate with the public. And, of course, the most notorious example of using the wrong units was the $125-million loss of the Mars Climate Orbiter in 1989 because someone was using pounds-force instead of Newtons. At school in the 60s, we used the poundal instead of the pound-force (on the rare occasions we used Imperial units). Perhaps this was because the staff were unwilling to teach a class of children to measure mass in slugs. Cheers, David.