Using the latest equipment and the mathematical process of triangulation to measure the meridian arc between these two sea-level locations, and then extrapolating the distance between the North Pole and the equator by extending the arc to an ellipse, the two astronomers aimed to meet back in Paris to come up with the new, universal standard of measurement within one year.
It ended up taking seven. The line of longitude used to determine the length of the metre runs through the centre of the Paris Observatory Credit: Madhvi Ramani. As Dr Alder details in his book, measuring this meridian arc during a time of great political and social upheaval proved to be an epic undertaking.
The two astronomers were frequently met with suspicion and animosity; they fell in and out of favour with the state; and were even injured on the job, which involved climbing to high points such as the tops of churches. The Pantheon, which was originally commissioned by Louis XV to be a church, became the central geodetic station in Paris from whose dome Delambre triangulated all the points around the city.
But despite all the technical mastery and labour that had gone into defining the new measurement, nobody wanted to use it. People were reluctant to give up the old ways of measuring since these were inextricably bound with local rituals, customs and economies. For example, an ell, a measure of cloth, generally equalled the width of local looms, while arable land was often measured in days, referencing the amount of land that a peasant could work during this time.
Eventually, in , Napoleon abandoned the metric system; although it was still taught in school, he largely let people use whichever measures they liked until it was reinstated in This was not just due to perseverance on the part of the state. Of course, it was tricky to do this unless you had clear, standard measures, such as the metre and the kilogram.
Originally established to preserve international standards, the BIPM promotes the uniformity of seven international units of measurement: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela. It is the home of the master platinum standard metre bar that was used to carefully calibrate copies, which were then sent out to various other national capitals. In the s, the BIPM redefined the metre in terms of light, making it more precise than ever.
And now, defined by universal laws of physics, it was finally a measure truly based on nature. The small, cylindrical weight cast in platinum-iridium alloy is also, like the metre, due to be redefined in terms of nature — specifically the quantum-mechanical quantity known as the Planck constant — by the BIPM this November.
As he explained the principle of the Kibble balance and the way in which a mass is weighed against the force of a coil in a magnetic field, I marvelled at the latest scientific engineering before me, the precision and personal effort of all the people who have been working on the kilogram project since it began in and are now very close to achieving their goal.
As with the 18th-Century meridian project, defining measurement continues to be one of our most important and difficult challenges. As I walked further up the hill of the public park that surrounds the BIPM and looked out at the view of Paris, I thought about the structure of measurement underlying the whole city.
The machinery used for construction; the trade and commerce happening in the city; the exact quantities of drugs, or radiation for cancer therapy, being delivered in the hospitals. What started with the metre formed the basis of our modern economy and led to globalisation.
It enabled high-precision engineering and continues to be essential for science and research, progressing our understanding of the universe.
We regret the error and have updated the text accordingly. By then, other countries had begun to adopt it, usually in the wake of political upheavals of their own. By the midth century, meters, kilometers and milliliters were standard units nearly the world over. But if you see something that doesn't look right, click here to contact us!
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