Fixing the imperial inch to the metre

摘要

Following my review of The Measure of All Things by Ken Alder (Geology Today, 2003. v.19, p. 186), a fascinating book on the birth of the metric system during the French Revolution, I have been interested in the relative conversion factor of inch to metre, particularly when Richard Gosnell drew my attention to a slight change in the metric conversion factor for the inch in old editions of Whitaker's Almanack. The interesting web pages of the National Physical Laboratory (http://www.npl.co.uk), the organization in charge of measurement and technical standards in the United Kingdom also provide information; David Flack at this organization kindly confirmed that the inch had been fixed at exactly 25.4 mm in July 1959. In this respect the Weights and Measures Act 1963 came into force on 31 January 1964. The international yard was redefined as 0.9144 metres, as until this time the US and British yards had different metric conversion factors. The Americans lost 2 millionths and the British inch gained 1.7 millionths of an inch to fix the international inch mid-way between the old measures. As such, it is worth looking again at this vexed question.