A Measure for all People, For All Time: The Story of the Metric System

Gabrielle Birchak/ May 27, 2025/ Enlightenment/ 0 comments

The French Revolution and the Birth of a Universal Measure

It’s 1789 in Paris. The French Rev­o­lu­tion is in full swing, heads are rolling (lit­er­al­ly), ideas of lib­er­ty and rea­son are elec­tri­fy­ing the air, and noth­ing is off-lim­its for reform. Amid this upheaval, one very prac­ti­cal prob­lem stood out: weights and mea­sures. Every region and every trade in France seemed to have its own units. A “pied” (foot) in one town could be a dif­fer­ent length than a foot in the next town. Length, weight, and vol­ume mea­sures were incom­pat­i­ble across France. This chaot­ic patch­work made trade and tax­a­tion a night­mare. Imag­ine buy­ing fab­ric where an “ell” is a dif­fer­ent length depend­ing on which city you’re in, or a farmer being cheat­ed because his bushel isn’t the same size as the merchant’s bushel. With their love of ratio­nal­i­ty, the rev­o­lu­tion­ar­ies were deter­mined to fix this.

Enter the French Nation­al Assem­bly, which in 1790 decid­ed that France (and hope­ful­ly the world) need­ed a com­plete­ly new mea­sure­ment sys­tem, one log­i­cal, uni­ver­sal, and based on nature. They offi­cial­ly com­mis­sioned the pres­ti­gious Académie des Sci­ences (French Acad­e­my of Sci­ences) to fig­ure it out. The politi­cians said, “Hey sci­en­tists, give us a sys­tem of mea­sures that will be the same every­where, and make it so good that we can say it is ‘for all peo­ple, for all time.’ No pressure!

A pan­el of five of France’s bright­est minds was appoint­ed to tack­le this in 1790. These were names for the ages: math­e­mati­cian Gas­pard Mon­ge, astronomer Pierre-Simon Laplace, the great Joseph-Louis Lagrange, thinker Nico­las de Con­dorcet, and naval sci­en­tist Jean-Charles de Bor­da. Quite the dream team of Enlight­en­ment sci­ence. Over the next year, they debat­ed how to cre­ate a per­fect mea­sure­ment sys­tem. And they pro­duced some guid­ing prin­ci­ples that still define the met­ric sys­tem today:

• Dec­i­mal base: Every­thing should be scaled by tens. No more 12 inch­es in a foot, 3 feet in a yard, or 16 ounces in a pound. Instead, units would neat­ly mul­ti­ply or divide by 10. It’s easy to calculate.
• Nat­ur­al units: The unit of length should come from nature, specif­i­cal­ly from the size of the Earth itself. They decid­ed that the pri­ma­ry unit of length, the meter, would be defined as one ten-mil­lionth of the dis­tance from the North Pole to the Equa­tor along the merid­i­an line run­ning through Paris. In oth­er words, take a quar­ter of Earth’s merid­i­an and chop it into 10 mil­lion equal pieces; one piece is a meter. A bold idea: lit­er­al­ly mea­sur­ing the Earth to define a unit! For weight, they said let’s use water as a ref­er­ence, and the unit of mass should be the mass of a cube of water of a spe­cif­ic size (since water’s some­thing every­one can access).
• Uni­ver­sal names and pre­fix­es: They even pro­duced a nam­ing scheme using Latin and Greek roots for dif­fer­ent scales. For exam­ple, kilo- for 1000, cen­ti- for 1/100, mil­li- for 1/1000, etc. So you could eas­i­ly tell how big or small a unit was. A kilo­me­ter is 1000 meters, a mil­li­liter is 1/1000 of a liter, and so on, much more straight­for­ward than 5280 feet in a mile or 14 pounds in a stone.

The French Nation­al Assem­bly loved these pro­pos­als and for­mal­ly approved this plan in 1791. Great! Now, they had to mea­sure that merid­i­an arc accu­rate­ly and make the pro­to­type meter and kilo­gram. Sim­ple, right? (Spoil­er: not that simple.)

Delambre and Méchain: The Meter’s Grand Adventure

To define the meter, they had to get a pre­cise mea­sure­ment of the dis­tance from the pole to the Equa­tor. Of course, they could­n’t mea­sure the whole quar­ter merid­i­an, but they could mea­sure a rep­re­sen­ta­tive chunk of it and extrap­o­late. So, they decid­ed to mea­sure the dis­tance along the Paris merid­i­an from Dunkirk, France (near the North Sea) all the way south to Barcelona, Spain. This stretch cov­ered a good swath of the Earth’s curvature.

Who drew the short straw for this assign­ment? Two astronomer-sur­vey­ors: Jean-Bap­tiste Delam­bre and Pierre Méchain. In 1792, Delam­bre and Méchain set out to sur­vey this merid­i­an arc through painstak­ing tri­an­gu­la­tion. And let me tell you, it turned into a six-year odyssey full of chal­lenges. Remem­ber, France in the 1790s was… how shall I put it… a bit unsta­ble. The Reign of Ter­ror was under­way; France was at war with mul­ti­ple Euro­pean pow­ers. It’s not exact­ly a relax­ing time for a road trip across the coun­try­side with strange sci­en­tif­ic instruments.

Delam­bre head­ed north­ward from Paris toward Dunkirk, and Méchain went south toward Barcelona. They used high-pre­ci­sion instru­ments to mea­sure angles between land­marks (church spires, hill­tops) to cre­ate tri­an­gles across the land, a method to cal­cu­late dis­tances. They care­ful­ly mea­sured base­line dis­tances using rods and then fig­ured out the rest trigono­met­ri­cal­ly. All while dodg­ing lit­er­al and fig­u­ra­tive bullets.

Local vil­lagers and troops often had no idea what these gen­tle­men were doing. Imag­ine two guys lug­ging giant brass instru­ments, climb­ing tow­ers, and send­ing flash­es of light or wav­ing flags across the hills. In that cli­mate, peo­ple sus­pect­ed they might be spies or up to no good. There’s an anec­dote that Delam­bre was chased, harassed, and even thrown in jail by local author­i­ties who thought his tri­an­gu­la­tion equip­ment was some kind of espi­onage toolk­it. Delam­bre wrote let­ters back to Paris, des­per­ate­ly say­ing, “Please send help, peo­ple keep arrest­ing me or break­ing my instru­ments!” It was that kind of trip.

Méchain, work­ing in Span­ish Cat­alo­nia, faced his own woes. At one point, the war between France and Spain halt­ed his progress. Méchain was also a per­fec­tion­ist and became obsessed with an anom­aly he detect­ed in his mea­sure­ments near Barcelona. He kept re-mea­sur­ing the lat­i­tude of Barcelona’s loca­tion because it didn’t pre­cise­ly match what he expect­ed. This quest for per­fect accu­ra­cy would haunt him. He even hid some data because he doubt­ed it.

Nev­er­the­less, by 1798 (after more than six gru­el­ing years in the field), Delam­bre and Méchain final­ly com­plet­ed their sur­vey and returned to Paris as sci­en­tif­ic heroes. One can only imag­ine the scene: they come back weath­er-beat­en and exhaust­ed but car­ry­ing note­books filled with num­bers that hold the key to the new meter. Using their data, the cal­cu­lat­ed length of the meter was final­ized in 1799. It turned out to be remark­ably close to what we know today: about 39.37 of our cur­rent inch­es long. If you want the exact fig­ure, the meter was defined then as 443.296 lignes in old French units, but we will stick to mod­ern terms.

To make the meter’s def­i­n­i­tion real, they cast a met­al pro­to­type bar. In June 1799, a bar of plat­inum was made to embody the new meter, which they called the “Mètre des Archives” and stored in Paris as a ref­er­ence. Like­wise, for mass, they made a plat­inum cylin­der to rep­re­sent the kilo­gram, defined as the mass of a liter of water at the tem­per­a­ture of melt­ing ice. This kilo­gram pro­to­type, known as the “Kilo­gramme des Archives,” was about 3.5 cen­time­ters tall and wide. It was a fan­cy, shiny weight that was sup­posed to equal the heft of one liter of water. These two objects, the meter bar and kilo­gram cylin­der of 1799, became the phys­i­cal stan­dards for the new system.

The sci­en­tists were opti­mistic, even utopi­an, about their cre­ation. The met­ric sys­tem was tout­ed with the mot­to “for all peo­ple, for all time.” They tru­ly hoped it would be adopt­ed world­wide and last for­ev­er since it was based on the Earth­’s size. Lit­tle did they know the Earth isn’t a per­fect sphere. There were slight flat­ten­ings at the poles, which meant their cal­cu­la­tions had tiny errors. Remem­ber Méchain’s obses­sive re-mea­sur­ing? He had found a slight dis­crep­an­cy in the data. It turned out their defined meter was ever so slight­ly off because Earth is not a per­fect sphere. It was a very minute dif­fer­ence, like a frac­tion of a mil­lime­ter. Still, it weighed on poor Méchain so much that he report­ed­ly con­fessed it only on his deathbed. Luck­i­ly, this tiny error was not enough to derail the project. As they defined it, the meter was close enough for gov­ern­ment work, and the met­ric sys­tem was offi­cial­ly born in France in 1795, with the final stan­dards in place by 1799.

By 1800, France had a new, incred­i­bly log­i­cal sys­tem of measurement.

Resistance and Revolution: The Metric System’s Bumpy Road to Adoption

But, if there’s one thing we humans love as much as a good idea, it’s not let­ting go of old habits. It turns out that con­vinc­ing every­day cit­i­zens and mer­chants to adopt the met­ric sys­tem was almost as chal­leng­ing as the geo­det­ic expe­di­tion itself! The French gov­ern­ment intro­duced the met­ric sys­tem as the law of the land in 1795, mak­ing it the offi­cial sys­tem of weights and mea­sures. How­ev­er, the imple­men­ta­tion was “poor­ly man­aged” by mod­ern stan­dards. They had to edu­cate an entire nation, much of it illit­er­ate, about new units with unfa­mil­iar names. The gov­ern­ment print­ed thou­sands of pam­phlets explain­ing the new mea­sures. They even pro­duced many phys­i­cal meter­sticks to dis­trib­ute. But it wasn’t enough. Paris alone need­ed half a mil­lion-meter sticks to sup­ply all the shop­keep­ers and cit­i­zens; only a few tens of thou­sands were ready in time.

Com­mon folks were under­stand­ably con­fused and skep­ti­cal. Imag­ine one day you’re told the arpent and toise you’ve used for­ev­er are no more, and now you must use meters and liters based on some arc of the Earth you’ve nev­er seen. The met­ric units had no ref­er­ence point in people’s dai­ly expe­ri­ence yet. It prob­a­bly felt like abstract gib­ber­ish to a farmer or a bak­er. And amid the tur­moil of the Rev­o­lu­tion, which was not exact­ly a calm time to intro­duce bureau­crat­ic changes, peo­ple had big­ger wor­ries, like sur­viv­ing. As a result, the met­ric sys­tem ini­tial­ly met a lot of resis­tance and even out­right non-com­pli­ance. Many folks kept right on using the old famil­iar units in mar­kets and homes, ignor­ing the new laws as long as they could.

Enter Napoleon Bona­parte, the famous gen­er­al who became the ruler of France. Napoleon inher­it­ed this met­ric sys­tem project when he took pow­er. And guess what? He wasn’t a fan at first. He report­ed­ly ridiculed the met­ric sys­tem for being too com­pli­cat­ed and incon­ve­nient for ordi­nary peo­ple. Napoleon, who was busy con­quer­ing half of Europe, prob­a­bly didn’t have patience for peo­ple com­plain­ing about how many cen­timètres were in a mètre. As a prag­mat­ic leader, he rec­og­nized the val­ue of a uni­form sys­tem. Still, he also saw that the pop­u­lace hat­ed the abrupt change. So, in 1812, Napoleon made a strate­gic retreat on the enforce­ment. He intro­duced cus­tom­ary mea­sures, essen­tial­ly a com­pro­mise allow­ing the old unit names back into day-to-day com­merce, but sly­ly rede­fined in terms of the met­ric sys­tem. For exam­ple, he brought back the livre (the old French pound) so bak­ers could go on sell­ing “a pound of bread,” but offi­cial­ly, one livre was now 500 grams. The old pied, the mea­sure­ment of a foot, was allowed again, defined as one-third of a meter. Peo­ple could use their famil­iar words, while under the hood, France was still metric.

This half-mea­sure (pun intend­ed) eased the tran­si­tion. Even after Napoleon fell from pow­er, France returned ful­ly to the prop­er met­ric sys­tem by 1840, mak­ing the use of those old names for met­ric units ille­gal and reaf­firm­ing that only the met­ric units were offi­cial. It took decades, but even­tu­al­ly, the French pub­lic got used to meters and kilo­grams. By the mid-nine­teenth cen­tu­ry, France was solid­ly met­ric in law and practice.

Beyond France, Napoleon’s armies had swept across Europe. In the process, they intro­duced the met­ric sys­tem to con­quered ter­ri­to­ries in the ear­ly 1800s. Some places, like parts of Italy, the Low Coun­tries, and Ger­many, adopt­ed it under French rule. After Napoleon’s defeat, a few revert­ed to old ways for a time, but the seed was plant­ed. Over the nine­teenth cen­tu­ry, one coun­try after anoth­er saw the appeal of a uni­ver­sal system:

• Por­tu­gal was an ear­ly adopter in 1814, which is sur­pris­ing since it wasn’t under French occupation.
• Latin Amer­i­can coun­tries, many of which were new­ly inde­pen­dent in the mid-1800s, like Chile, adopt­ed the met­ric system.
• Italy and Ger­many found the met­ric sys­tem use­ful dur­ing their nation­al uni­fi­ca­tions in the 1860s; it was a neu­tral, mod­ern sys­tem that helped uni­fy region­al differences.
• By 1875, the met­ric system’s suc­cess led to a land­mark inter­na­tion­al agree­ment: the Treaty of the Meter. Sev­en­teen nations (includ­ing sig­nif­i­cant pow­ers like Britain, the U.S., Ger­many, and, of course, France) signed this treaty to coop­er­ate in main­tain­ing stan­dard mea­sures. They estab­lished the Inter­na­tion­al Bureau of Weights and Mea­sures (BIPM), which was a sort of “met­ric HQ,” to keep the pro­to­type meter and kilo­gram and over­see com­par­isons. This treaty made the met­ric sys­tem a tru­ly glob­al con­cern, even if some coun­tries were slow to imple­ment it at home.

Notably, Great Britain, the inven­tor of the rival Impe­r­i­al sys­tem, was a tough nut. The British hes­i­tat­ed to adopt “that French sys­tem” for a long time. Nation­al pride dies hard; British indus­try and trade were deeply invest­ed in their own units. The Unit­ed King­dom only offi­cial­ly began met­ri­ca­tion in the late twen­ti­eth cen­tu­ry, and today, it still uses a mix of mea­sure­ments, like pints of beer and miles on road signs. How­ev­er, sci­ence and engi­neer­ing indus­tries use the met­ric system.

And the Unit­ed States? Why is the U.S. still not ful­ly met­ric? The U.S. par­tic­i­pat­ed in the 1875 Treaty of the Meter. It even adopt­ed laws in 1866, mak­ing it legal to use the met­ric sys­tem. Amer­i­can sci­en­tists and indus­tries use met­rics exten­sive­ly. How­ev­er, the U.S. nev­er man­dat­ed a met­ric sys­tem for every­day use; pounds, inch­es, and gal­lons remained king cul­tur­al­ly. There was a push in the 1970s to met­ri­cate, but it fiz­zled out. Today, the U.S. stands almost alone in cling­ing to cus­tom­ary units in dai­ly life, along with only a cou­ple of oth­er coun­tries that haven’t ful­ly con­vert­ed. Myan­mar and Liberia were the oth­er two hold­outs for years, though they’ve also expressed intent to go met­ric. Still, even Amer­i­cans deal with met­ric units more than we real­ize with 2‑liter soda bot­tles, 100-meter dash­es, 9mm bul­lets, and 5K runs.

By the 20th cen­tu­ry, the met­ric sys­tem had grown from a rev­o­lu­tion­ary French idea into the world’s stan­dard for sci­ence and inter­na­tion­al trade. Near­ly all coun­tries have adopt­ed it offi­cial­ly, at least for com­merce and sci­ence. It tru­ly lived up to being a sys­tem “for all peo­ple,” not just the French. But what about “for all time”? To answer that, we need to see how the met­ric sys­tem evolved over time to become even more pre­cise and robust.

Modern Marvels: How the Metric System Powers Today’s World

The met­ric sys­tem, now known as the Inter­na­tion­al Sys­tem of Units (SI), is the silent back­bone of our high-tech world. From space trav­el to AI and dig­i­tal infra­struc­ture, it’s the uni­ver­sal lan­guage that keeps glob­al sys­tems aligned. One of the most infa­mous reminders of why stan­dard­iza­tion mat­ters came in 1999 when NASA lost the Mars Cli­mate Orbiter due to a mix-up between impe­r­i­al and met­ric units, a $125 mil­lion mis­take. Today, agen­cies like NASA, the ESA, and oth­ers rely on met­ric stan­dards to ensure com­pat­i­bil­i­ty and pre­ci­sion. From oxy­gen lev­els dur­ing space­walks to bolt sizes on the Inter­na­tion­al Space Sta­tion, every­thing is mea­sured in meters, liters, and kilo­grams to make inter­na­tion­al col­lab­o­ra­tion possible.

The met­ric sys­tem enables seam­less data inte­gra­tion in arti­fi­cial intel­li­gence and glob­al sci­ence. Whether it’s cli­mate data or med­ical research, using kelvins, pas­cals, and mil­ligrams elim­i­nates the con­fu­sion of con­ver­sion errors. AI sys­tems in med­i­cine, robot­ics, and autonomous vehi­cles rely on sen­sors that mea­sure in met­ric units, mak­ing it eas­i­er for tech­nolo­gies made in dif­fer­ent coun­tries to work togeth­er. Even com­put­er chips are man­u­fac­tured with nanome­ter pre­ci­sion. Emerg­ing tech like quan­tum com­put­ing and neur­al net­works depend on mea­sure­ments ground­ed in microkelvins and met­ric-defined constants.

Beyond sci­ence, the dig­i­tal econ­o­my and every­day life are steeped in met­ric log­ic. Bit­coin uses denom­i­na­tions like mil­li­bit­coin and micro­bit­coin, mir­ror­ing SI pre­fix­es. Inter­net speeds are mea­sured in megabits per sec­ond, hard dri­ves in giga­bytes, and glob­al time­keep­ing runs on the SI sec­ond, syn­chro­niz­ing satel­lites, servers, and finan­cial sys­tems world­wide. The met­ric sys­tem, once envi­sioned dur­ing the French Rev­o­lu­tion, has become an endur­ing frame­work that sup­ports every­thing from Mars land­ings to the data pow­er­ing your smart­phone. Its clar­i­ty, con­sis­ten­cy, and uni­ver­sal­i­ty con­tin­ue to unite our increas­ing­ly inter­con­nect­ed world.

The his­to­ry of the met­ric sys­tem is more than a sto­ry of mea­sure­ments, it’s a sto­ry of human ambi­tion, rea­son, and the pur­suit of uni­ty. Born out of Rev­o­lu­tion and Enlight­en­ment ideals, it set out to cre­ate a sys­tem “for all peo­ple, for all time.” Remark­ably, it suc­ceed­ed. From the per­ilous expe­di­tions of Delam­bre and Méchain to today’s quan­tum-ref­er­enced def­i­n­i­tions, the met­ric sys­tem has evolved into a glob­al foun­da­tion for sci­ence, tech­nol­o­gy, and com­mu­ni­ca­tion. It reminds us that shared stan­dards can fos­ter col­lab­o­ra­tion and progress even in a divid­ed world. So, whether you’re cal­cu­lat­ing a recipe, launch­ing a satel­lite, or decod­ing a genome, you’re part of a lega­cy that began with a bold idea: the entire world could agree on how to mea­sure it.

Until next time, carpe diem!

---

Sources

BIPM (Bureau Inter­na­tion­al des Poids et Mesures). The Inter­na­tion­al Sys­tem of Units (SI). Sèvres, France: BIPM, 2019. (All SI units defined in terms of fun­da­men­tal con­stants as of 2019) (SI Rede­f­i­n­i­tion | NIST).

Bosak, Steven. “His­to­ry of the Met­ric Sys­tem.” Met­ric Met­al (Park­er Steel Com­pa­ny Blog), n.d. https://www.metricmetal.com/history-of-the-metric-system/. (Overview of met­ric sys­tem ori­gins and adop­tion, includ­ing 1790 French ini­tia­tive, sur­vey expe­di­tion chal­lenges, and glob­al adop­tion time­line) (His­to­ry of the Met­ric Sys­tem | Met­ric Met­al) (His­to­ry of the Met­ric Sys­tem | Met­ric Met­al).

Gem­i­ni. “Cryp­to Denom­i­na­tions Explained: BTC & Satoshis; ETH & Gwei.” Gem­i­ni Cryp­to­pe­dia, updat­ed Feb­ru­ary 26, 2025. https://www.gemini.com/cryptopedia/satoshi-value-gwei-to-ether-to-wei-converter-eth-gwei. (Explains Bit­coin units and notes their met­ric-style nam­ing con­ven­tions) (Cryp­to Denom­i­na­tions: Satoshi Val­ues & Wei to Ether | Gem­i­ni).

NASA. “Mars Cli­mate Orbiter Mishap Inves­ti­ga­tion Board, Phase I Report.” NASA Jet Propul­sion Lab­o­ra­to­ry, Novem­ber 10, 1999. (Details of Mars Cli­mate Orbiter fail­ure due to metric/U.S. cus­tom­ary unit mix-up) (Mars Cli­mate Orbiter — Wikipedia).

Robin­son, Andrew. “His­to­ry: Scal­ing Up.” Nature 478, no. 7367 (Octo­ber 2011): 32–33. (Review of the his­to­ry of met­ri­ca­tion, not­ing Napoleon’s changes in 1812 and lat­er adop­tion in Europe) (Met­ri­ca­tion — Wikipedia) (Met­ri­ca­tion — Wikipedia).

Smith­son­ian Insti­tu­tion. “The Ori­gin of the Met­ric Sys­tem.” Nation­al Muse­um of Amer­i­can His­to­ry, Met­ric Sys­tem Demon­stra­tion Appa­ra­tus. Accessed April 2025. https://www.si.edu/spotlight/metric-system-demonstration-apparatus/the-origin-of-the-metric-system. (Describes the French Revolution’s role in cre­at­ing the met­ric sys­tem and its ear­ly fea­tures) (The Ori­gin of the Met­ric Sys­tem | Smith­son­ian Insti­tu­tion).

Smith­son­ian Insti­tu­tion. “Dif­fus­ing and Oppos­ing the Met­ric Sys­tem.” Nation­al Muse­um of Amer­i­can His­to­ry. Accessed April 2025. https://www.si.edu/spotlight/metric-system-demonstration-apparatus/diffusing-and-opposing-the-metric-system. (Cov­ers 19th-cen­tu­ry efforts to pro­mote met­ric sys­tem and resis­tance in var­i­ous countries).

Tier­ney, John. “Bust­ing Myths about the Met­ric Sys­tem.” NIST Blog, Nation­al Insti­tute of Stan­dards and Tech­nol­o­gy, July 27, 2020. (Clar­i­fies com­mon mis­con­cep­tions; notes only U.S., Myan­mar, and Liberia lack manda­to­ry met­ric adop­tion in dai­ly life) (Only the U.S., Liberia and Myan­mar are “not met­ric”?).

Wikipedia. “His­to­ry of the Met­ric Sys­tem.” Last mod­i­fied Jan­u­ary 14, 2025. https://en.wikipedia.org/wiki/History_of_the_metric_system. (Com­pre­hen­sive his­to­ry from ear­ly pro­pos­als to mod­ern SI, includ­ing the 1790s French merid­i­an expe­di­tion and adop­tion events) (His­to­ry of the met­ric sys­tem — Wikipedia) (His­to­ry of the met­ric sys­tem — Wikipedia).

Wikipedia. “Mars Cli­mate Orbiter.” Last mod­i­fied March 28, 2025. https://en.wikipedia.org/wiki/Mars_Climate_Orbiter. (Inci­dent sum­ma­ry of the Mars probe lost due to a units error, with met­ric vs impe­r­i­al detail) (Mars Cli­mate Orbiter — Wikipedia).

Wikipedia. “Met­ri­ca­tion.” Last mod­i­fied Feb­ru­ary 3, 2025. https://en.wikipedia.org/wiki/Metrication. (Infor­ma­tion on how and when var­i­ous coun­tries adopt­ed the met­ric sys­tem; Napoleon’s mesures usuelles and 19th-cen­tu­ry met­ri­ca­tion mile­stones) (Met­ri­ca­tion — Wikipedia) (Met­ri­ca­tion — Wikipedia).

Wikipedia. “Plat­inum-Irid­i­um Meter Bar (Inter­na­tion­al Pro­to­type Metre).” Wiki­me­dia Com­mons, image by NIST (pub­lic domain), accessed April 2025. (Image of the 1889 Inter­na­tion­al Pro­to­type Metre bar) (File:Platinum-Iridium meter bar.jpg — Wiki­me­dia Com­mons).

Wikipedia. “Inter­na­tion­al Pro­to­type Kilo­gram (Le Grand K).” Wiki­me­dia Com­mons, image by BIPM (CC BY-SA 3.0 IGO), accessed April 2025. (Image of the plat­inum-irid­i­um kilo­gram arti­fact under bell jars) (File:International pro­to­type of the kilo­gram aka Le Grand K.jpg — Wiki­me­dia Com­mons).