Measured response

Frank McManus ("Letter of the Week", 11 March) is being a bit hard on the metre, which was originally intended to be one-ten-millionth of the distance from the pole to the equator - arbitrary, if you like, but scarcely arcane. As a more convenient standard was needed, it was agreed that a definition based on the distance between two marks engraved on a metal bar should be used, the bar being held at the BIPM (International Bureau of Weights and Measures) in Sevres, with secondary standards of a similar type to be kept at each of the main national measurement laboratories.

But comparisons between the various standard bars entailed transporting them from one laboratory to another; to avoid such an inconvenience and the risk of damage, the definition quoted by McManus was eventually agreed on.

However, this definition has been obsolete for a number of years, and the current definition is "the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom".

H Pursey
New Malden, Surrey

This article first appeared in Far from the Promised Land

2002-03-18