Johansson and the inch[edit]

In the 1910s, the U.S. and U.K. definitions of the inch differed, with the U.S. inch being defined as 25.4000508mm (with a reference temperature of 68 °F (20 °C)) and the U.K. inch at 25.399977mm (with a reference temperature of 62 °F (17 °C)). When he started manufacturing gauge blocks in inch sizes in 1912, Johnanson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johannson's blocks were so popular, his blocks became the de facto standard for manufacturers in both countries. When the inch was eventually redefined to be exactly 25.4mm in both the U.K. (in 1930) and U.S. (in 1933),[4] this effectively endorsed what was already common practice worldwide.[5][6]

- bill 9-17-2021 9:45 pm

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