Earth and Space

Standardising units in Revolutionary France – a scientist’s anxiety and the mistaken metre

Stories from Physics for 11-14 14-16 IOP RESOURCES

It is estimated that 250,000 different weights and measures were in use across pre-revolutionary France.

In 1791, the National Assembly attempted to standardise length measurement by defining the metre as one ten-millionth the distance from the equator to the North Pole. Astronomers Pierre Méchain and Jean Baptiste Joseph Delambre were commissioned to measure the meridian between Dunkirk and Barcelona. The surveyors would measure the distance by marking out a hundred triangles, with bases of approximately 36,000 ft, measured by moving standard 12 ft platinum rods over 3000 times.

But in the unrest that followed the revolution, the scientists’ specialised equipment sparked local peoples’ suspicions and both Méchain and Delambre were arrested as potential counter-revolutionaries.

Though freed from arrest, Méchain’s work did not go well – he broke his arm and ribs in an accident and he began to notice inconsistencies in his data driving him into depression. He wrote to Delambre: ‘After all that has happened I can no longer show myself anywhere and my only wish is to be annihilated’.

Though he initially resisted, Méchain was persuaded to return to Paris to report his findings to a conference, but whilst Delambre presented his data, Méchain refused. Assuming the French scientists had fabricated their research, a Danish delegate left the conference in disgust. Nonetheless, the conference organisers declared the scientists’ data sound and used them to calculate the provisional length for the metre. Prototypes were produced based on the measurements, leading to the famous platinum standard rod in 1799.

Despite the honours and awards that followed the conference, Méchain continued to be plagued by doubts over his measurements and sought to collect additional data. In carrying out this research, he died from yellow fever.

Delambre received his colleague’s papers after his death and found them to consist of a mess of undated, unbound pages covered in crossings out, and he had to clean up the data before it could be published.

A quarter of a century after Méchain’s death, an astronomer analysing his papers found that Méchain had made mistakes and deliberately changed his data. But these distortions were minor in comparison to measurement uncertainties.

By the 1870s, the error in the measurement of the metre was well known and a debate arose between those who felt the measurement should be changed to its ‘true’ value and those in favour of continuity. The conservative faction won, arguing that changing the measurement would have led to international disruption.

The result is that the metre used today is two parts in 10,000 shorter than the ideal of one ten-millionth the distance from the equator the North Pole (contemporary satellite data show this distance to be 10,002,290 m). Doubtless Méchain would be appalled to know that his error has persisted for centuries.


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