Drop your feet and use your fingers

Sentiment apart, counting in tens has got to be easier, argues Tom Wilkie Changing how we measure things is heavy with political meaning, says Tom Wilkie

Across the country tomorrow, in corner shops and Sainsbury's Homebase, the cry will resound: "Let us have Magna Carta and the Iron Yard of Our Lord, King John". For this weekend, amid much predictable anguish, metric measures for pre-packed food and planks of wood will be compulsory by law.

Lengths of wood can now legally be sold only in multiples of "the length of the path travelled by [a beam of] light in a vacuum during a time interval of 1/299792558 of a second". A second is "the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom". So as youshop tomorrow, demand to see the cesium-133. Don't let anyone try to fob you off with the cesium-137 atom - you'll be getting short measure.

It is easy to mock. But weights and measures have a significant connection with political revolution. It was thought important enough for the Magna Carta that the King should specify uniform weights and measures for England. A few years later, the royal "Assize of Weights and Measures" established the Iron Yard of our Lord the King as the standard measure, until the Tudor coup d'etat of Henry VII, who promulgated new standards in 1496.

But the Imperial system is boring compared to the metric story, tinged with heroism, romance and tragedy. New standards were promoted by Talleyrand in April 1790, but it took nine years for France to adopt the system. Its devisers set out to find a truly "natural" measure, by relating the metre to the earth's circumference, finally fixing it as one ten-millionth of the estimated distance between the North Pole and the equator.

In 1792, Louis XVI of France issued a proclamation that led to the fixing the length of the metre. The unfortunate Louis was in prison at the time, fated to die shortly thereafter by guillotine. But his decree empowered two engineers, Jean Delambre and Pierre Mechain, to conduct the precision measurements of the distance on the meridian from Barcelona in Spain to Dunkirk on the Channel coast, from which they could extrapolate the entire circumference. Their survey was interrupted by civil war and foreign invasion and the engineers risked death or imprisonment as revolutionary peasants mistook them for spies. It took six years to complete their survey - something to think of at Homebase tomorrow, perhaps.

The metric system was gradually superseded by the International System of Units and it is the precision of modern scientific measurement that we have to thank for the somewhat abstract official definitions today. The only homely touch left is the standard kilogramme, a cylinder of platinum-iridium metal alloy kept by the International Bureau of Weights and Measures at Sevres near Paris. It is taken out to cross-calibrate against secondary standards only rarely. Every time it is moved, there is a danger that a few atoms might rub off, so the kilogramme might be getting lighter with the years. If it is exposed to the air, there is a risk that the platinum might oxidise slightly, binding a few oxygen atoms to the cylinder and making it heavier. We would never know which is happening, because it is the standard kilogramme and by definition cannot weigh anything more or less than one kilogramme.

In the end, it is not the size of the units but how they are counted that gives them their naturalness. We have 10 fingers and therefore count in tens. The utility of the metric system is that we count metres and grammes in units, tens, hundreds, thousands and so on just as we do for numbers. It is economical and efficient. The Chinese realised this in 600BC when they adopted a decimalised system.

And anyway, by act of parliament in 1963, all English weights and measures were re-defined in terms of the metric system. We no longer have an Iron Yard, nor even that bronze bar supported on bronze rollers envisaged by an 1878 Act of Parliament where the standard yard was "the straight line or distance between the centres of two gold plugs or pins in the bronze bar". For more than 30 years, the standard yard has been defined as exactly 0.9144 metres. We have been subject to the metric system all this time anyway. As the millennium approaches, perhaps it really is time to let the 18th century wholeheartedly into our shops and DIY superstores.