Ye Olde Inches

[graham-xrf]

I found it on Wikimedia Commons. This inch converter is in the Museum of the History of Science in Oxford.
So OK, in the historic past, they did not manage to sync up very well. The one I find really good is to compare the Moscow Inch to the (huge) Russian Inch. Clearly Moscow folk needed more of them, or conversely, the country peasants in the rest of Russia needed their inches to go further - or they were being short-changed big-time!
I had expected better agreement within Russia!



Standards get there only with the coercive force of law, by Kings, Emperors, and Republics. In the case of metric, it was by the French Revolution. I love that there are French "Inches in there, right next to "Fr Metre", at 2.54cm to the English inch immediately below. I guess that at some stage, the "English Inch" was forced to get a bit bigger, because we know the platinum Mètre des Archives did not get any smaller!

They had lathes and machine tools everywhere well before then. Sync between nations? Maybe not so much!

[Edit The modern measured comparisons are here --> https://commons.wikimedia.org/wiki/File:Inch_converter.jpg ]

 

[benmychree]

If three barleycorns established the British inch, I wonder what established all those other inches? Or did they take the length of the king's foot and divide by twelve?

 

[NortonDommi]

Bloody French! Started this crazy driving on the wrong side of the road as well just to spite the British and America followed suite.
Here is an article by a friend of mine on weights, measures and volumes: http://www.celticnz.co.nz/Weights_Measures_Volumes/Weights_Measures.htm And let us not forget the reduced inch:

 

[graham-xrf]

If three barleycorns established the British inch, I wonder what established all those other inches? Or did they take the length of the king's foot and divide by twelve?

Hee Hee. It was always attempted to have it 1/36 of a yard, but yes, it was once linked to 3 barley corns placed end to end. That was from King Edward II. Easy enough to look up the tawdry details of arguably the worst most incompetent King in history, forced to abdicate to his son Prince Edward, and he came to a bad end.

King David I of Scotland approved the width of a man's thumb at the base of the nail, and tried getting the average of several men of different sizes.

American inches are metric!
Set to exactly? (Well nearly) 2.54cm since 1866, by having 1 metre be 39.37 inches. (Ans = 2.54000508). It saved the trouble and expense of establishing and maintaining an American artifact standard.. Johansson's gauge blocks were made so well, so many everywhere, so exactly alike to within a few millionths, kind of forced things along. I don't know if this was in the cause of making guns with interchangeable parts, but I guess it helped. 2.54cm exactly was adopted as late as 1930's. Sync happened in 1964' with US inch shrinking a couple of millionths, English inch getting bigger by not quite as much, and everybody's thous, and ammo, and P51 engines still worked regardless! My South Bend 9A is 1947, but I am sure 2.54cm Jo-Blocks were helping it have good inches.

The clock-makers have finally won!
Since 2019, all standards are no longer based on artifacts - especially barleycorns.
The speed of light is it! Exactly 299792458 metres in a second. Arrgh! that's a speed! It relies on knowing what is a second. We need a good clock!

The older USA clock atomic clock standard used to be to about one second in 300 million years. There are competing Chinese and German clocks, but the latest ones are so accurate that the maker's are not even sure their actual accuracy can be properly tested. The latest NIST clocks to parts of 1E-18. Less than a second in a time far past the known age of the Universe! The ability to measure seconds is now limited by the gravitational effects of the Earth.

All our other units have been messed with!
Since 2019, all are defined in abstract manner, linking them to constants. Mass is defined via Planck's Constant. Current is based on an exact number of electron charges per second.

Even the most dedicated of HM scrapers would be happy to get stuff within about 5 ten-thousandths, but now, the standards makers use interferometer methods down to about 200 atoms worth.

Way back among the nuts, the ultimate reference for mass via Planck's Constant is a fabulous bit of NIST jewelery called a "Kibble balance".


Imagine machining some of those bits!