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What is your height? This seemingly innocuous question, if you come to think of it, doesn’t always hand you the same answer. No, I’m not talking about the numerical part of the answer that is bound to be different for different people owing to their different heights. I am, instead, drawing your attention to the units that follow the numerical part of the answer.
For many of us, that answer could be in feet and inches. The rest of us, meanwhile, would stick to metres, the international system of units (Systeme International d’Unites, SI). This difference was at the heart of the birth of the metre, especially during a time when there were no standards.
Need for standardisation
We live in a hyperconnected modern world where the accurate measurement – be it of length, weight, or anything else – and precise definition are non-negotiables. From the smallest of electronic gadgets to the largest of buildings or transportation systems, global economies today function in a way such that items that make up the whole are sourced from around the world. For this earthly jigsaw puzzle to function at scale, the interchangeability and reproducibility has to be a given as the pieces have to fit perfectly every time, right from the first time.
We take this for granted now as we live in a time when the standards have been established beyond doubt and accuracy of measurement is probably at its best through history. Things, however, were rather different earlier, even if we were to dial back a little over 230 years and head to a time late in the 18th Century.
French connection
History-buffs out there will surely know the French Revolution as one of the biggest political, societal and historical events in history. What many might not know is the fact that it also served as the origin of the metric system.
Once the revolution was underway in 1789, the French public demanded uniform weights and measures everywhere in the nation in order to facilitate trade and avoid confusion. There was a clamour to replace the diverse regional systems of Europe with a new universal decimal system that was founded upon some “natural and invariable base”. The origin of the metric system was thus intertwined with the birth of the metre.
By 1790, this project was green flagged by the National Assembly and a basic unit of length measurement, the metre, was proposed. Some of the best minds of the age were involved in defining it and they chose one that was based on the quarter meridian – a metre was to be a decimal unit one ten-millionth of the distance between the terrestrial pole and the equator.
Also read: Greenwich, the historic home of the prime meridian
Measuring expedition
French mathematician, astronomer, historian and geodesist Jean Baptiste Joseph Delambre and fellow mathematician, astronomer and surveyor Pierre Francois Andre Mechain embarked on an expedition from Paris. Their objective was to establish the definitive length of the metre by taking geodetic measurements between Dunkirk and Barcelona as it formed an arc section of the meridian that passed through Paris. This arc measurement, which took them the better part of six years from 1792-98, thereby served as the basis for the very first definition of the metre.
Even as Delambre and Mechain were triangulating their way towards an accurate measure, the French National Assembly, on August 1, 1793, “affirmed the decimal system and the meridianal definition of the meter, ordered the continuation of the work, and decreed that the Academy provide for the manufacture, distribution, and explanation of provisional meters for general use while it prosecuted its measurements.”
Following the survey of the arc, three platinum standards and several iron copies of the metre were made. Additionally, in order to help the populace familiarise with the new measure, it was decided to place the standard in the busiest places. More than 10 were installed in Paris alone and a couple of them have even survived to this day.
Metre is redefined
In case you were under the impression that that was it, it wasn’t. For decades later, it was revealed through subsequent examinations that Delambre and Mechain had erred in their measurement. Given that many years had since passed, the metre was redefined rather than alter its length. Therefore, instead of a metre being one ten-millionth of the distance between the terrestrial pole and the equator, it was redefined as the distance between the two marks on a bar.
The 1875 Treaty of the Meter, or the Meter Convention, was signed on May 20 to “assure the international unification and improvement of the metric system”, helping create the International Bureau of Weights and Measures (French: Bureau international des poids et mesures, BIPM). The BIPM, a diplomatic organisation tasked with the responsibility of maintaining an international system of units, continues to facilitate the standardisation of measurements world-wide to this day. In its 151 years of existence, the definition of the metre has been altered a number of times to the current version that we now learn at schools.
The many metres
Here are the different definitions of the metre over time.
First definition (1793 onwards): One ten-millionth of the distance from the equator to the North Pole, measured along a meridian passing through Paris.
Prototype-based definition (1889 onwards): Once the error in the meridian measurement was identified, the metre was redefined as the distance between the two marks on a platinum bar that had served as the prototype until then.
Closeup of National Prototype Meter Bar made in 1889 by the BIPM.
| Photo Credit:
Wikimedia Commons
In 1889, a new prototype meter was created and internationally adopted. This was an improved platinum-iridium bar with lines inscribed at each end and the distance between them defining the metre.
Replicas of this prototype that were accurate, but not identical, were then created to disseminate the length standard to various national laboratories. While this definition afforded stability, there was still the dependence on a physical reference.
Atomic and optical standards (1960 onwards): The 1960 definition of a metre was based on a krypton-86 radiation from an electric discharge lamp. As the metre was redefined in terms of a specific number of wavelengths emitted by krypton-86 atoms, it marked the departure from definitions that were dependent on physical objects. Instead, the definition was now based on natural phenomena.
Speed of light definition (1983 onwards): The metre was further redefined in 1983 as the length of the path travelled by light in a vacuum during 1/299,792,458 of a second. By linking the metre with the speed of light, the unit of length was being linked to a fundamental constant of nature.
In 2002, a clarification was provided stating that the metre represents a unit of proper length. The most recent revision came in 2019 when the definition was rephrased to explicitly incorporate the definition of the second based on the caesium-133 atomic frequency.
Also on August 1: Andrew Smith Hallidie conducted a successful demonstration of a cable car in San Francisco in 1873
Published – August 01, 2026 07:57 am IST
