The metre began as a slice of the Earth. In June 1792 two French astronomers left Paris in opposite directions to measure the meridian arc running from Dunkirk through Paris to Barcelona. From that arc they meant to work out one ten-millionth of the distance from the North Pole to the equator, and call it a metre.
Physics World describes the same aim from the other end. The Académie des Sciences devised the new system, and it tied the metre to one forty-millionth of the Paris meridian, a full circle round the planet. The founders wanted a measure “for all times, for all peoples”.
Why France wanted a new unit
France had a measurement problem. The Guardian reported an estimate that the country used some 250,000 different units of weight and measure, under about 800 names. Units changed between provinces, between districts and sometimes between neighbouring towns.
The revolutionaries did not want to tidy that system. They wanted to replace it with one fixed to nature, according to Robert P Crease writing in Physics World.
Seven years on the road
Jean-Baptiste-Joseph Delambre took the northern stretch and Pierre-François-André Méchain took the southern one. Each reached his end of the arc and then measured his way back towards the other.
The work took seven years, through a country at war with itself and its neighbours. Delambre came close to the guillotine on the edge of Paris. Méchain was caught behind enemy lines in the war with Spain and imprisoned. The two finally met at Carcassonne, then took their data to an international commission in Paris.
The result became a bar of pure platinum. Physics World dates the platinum metre and its matching kilogram to 1799, when both went into the National Archives. Napoleon Bonaparte declared that conquests would come and go but this work would endure.
The error Méchain hid
The historian Ken Alder found a flaw in the survey while reading the expedition’s log books at the Paris Observatory. Méchain had taken readings from one spot in two different summers, and they did not agree. He kept the discrepancy to himself, and his letters show how badly it tormented him. He died of malaria on a later expedition that he hoped would put it right.
Delambre knew too. In a note in one of Méchain’s log books, he explained that he had kept back details that might shake public confidence in the mission. Alder set out the story in his book The Measure of All Things.
Modern satellite surveys put the pole-to-equator distance at 10,002,290 metres. Against that figure, the metre of 1799 comes out about 0.2mm short. Alder did not blame Méchain alone. He told the Guardian that the whole survey rested on the false assumption that the Earth is uniform, when in fact it is lumpy and warped.
What happened to the metre next
The link to the planet did not last. Physics World notes that keeping the unit tied to a natural standard proved impractical, so the platinum bar in the archives became the definition instead.
The Treaty of the Metre in 1875 moved oversight of the system from France to the International Bureau of Weights and Measures. That treaty led to a new International Prototype of the Metre, made in 1879 and adopted in 1889. Its makers calibrated it against the old archive metre, so the survey’s figure carried straight through.
In 1960 the metre stopped depending on any bar. It was redefined by the light from an optical transition of krypton-86. In 1983 it changed again, to a definition based on the speed of light. Alder argued that the 0.2mm shortfall from the 1790s survives in every one of those definitions. The prototype bar sits in a vault at the bureau, a historical curiosity.
