
More than 5000 entries on the history, culture and life of Britain (published in 1993 by Macmillan, now out of print)

metric system


The system which in the late 20C is gradually replacing *imperial weights and measures in Britain. It is based on a supremely logical concept first put forward in France in 1670 (by the vicar of a church in Lyons) and eventually introduced by the National Assembly during the Revolution (in 1791). The essence of the system is not only a decimal relationship between each grade of units; it is that a single unit was made the basis for measurements of length, weight and volume. The unit chosen was the metre, defined as one tenmillionth of the distance from the equator to the north pole.




The unit of weight, the gram, was then defined as the weight of a cube of water measuring one hundredth of a metre in each dimension; and the unit of liquid volume, the litre, was defined as the amount which would fit in a cube one tenth of a metre all round (or, stated differently, the space occupied by one kilogram of water, which amounts in this elegant edifice to the same thing). With improved techniques of measurement the definition of the metre has changed – it is now 1,650,763.73 wavelengths of the orangered line of krypton86 – but by 1791 the system itself was in place.



