SPEAKER: Jim Grozier (University College, London)
In June 1792, two of France’s foremost astronomers set out from Paris with six assistants and some of the most advanced surveying equipment available. Their mission: to measure the length of the Paris meridian from Dunquerque to Barcelona. The previous year, the French Assembly had defined the new unit of length, the metre, as one ten-millionth of the distance from the North Pole to the Equator, and this could be quantified by measuring a sizeable fraction of that distance and scaling up. Along with that of the new unit of weight – the grave or kilogram – the definition of the unit in terms of a “natural” quantity, the circumference of the earth, was seen as a break from previous practices involving arbitrary physical standards.
The expedition was supposed to take a year, but the astronomers did not return to Paris until 1798. On their way, in a country lurching from one political crisis to another, they encountered all manner of hazards and hardships. I will give an overview of the story of their odyssey – which has been well told in Ken Alder’s book The Measure of All Things – but take a more detailed look at the technical aspects of the measurements they made and the way they were processed, up to and including the final announcement of the length of the metre in terms of the old unit, the toise.
Unlike the ten-day week and the 400-degree circle, the metre survived France’s post-revolutionary backlash, and was adopted by several other countries. In 1875 it was redefined and became a truly international unit, but the link to “nature” was lost: the new metre was simply the length of the standard, the International Prototype Metre, which in turn was decreed to be the same length as its predecessor. But Nature had the last laugh: in 1960, the metre was redefined again, this time in terms of a truly “natural” quantity, independent of even planetary dimensions – the speed of light. This set the scene for other units to be defined in terms of fundamental constants, and at the 26th meeting of the General Conference for Weights and Measures in November 2018, it is expected that four of the SI base units will be similarly redefined in terms of fundamental constants, so that there will be no more need for primary standards.
Image: US National Institute of Standards and Technology (NIST)