Trips Places Foods Stories Newsletters
Loading...
Loading...
Loading...
Loading...

No search results found for
“”

Make sure words are spelled correctly.

Try searching for a travel destination.

Places near me Random place

Popular Destinations

  • Paris
  • London
  • New York
  • Berlin
  • Rome
  • Los Angeles
Trips Places Foods Stories Newsletters
Sign In Join
Places near me Random place
All France Saint-Cloud Le Grand K

Le Grand K

The former global standard for the exact mass of one kilogram sits under lock and key outside Paris.

Saint-Cloud, France

Added By
Tony Dunnell
Email
Been Here
Want to go
Added to list
CAPTION
Computer-generated image of Le Grand K.   Greg L/cc by-sa 3.0
A replica of Le Grande K.   National Institute of Standards and Technology/public domain
International Bureau of Weights and Measures in Paris.   Olevy/cc by-sa 4.0
A replica of Le Grande K at the Cité des Sciences et de l’Industrie, Paris.   Japs 88/cc by-sa 3.0
Copy number K4, kept by the U.S. National Institute of Standards and Technology.   U.S. government/public domain
Kilogram No. 20 (right) was a copy of the International Prototype Kilogram, used in the U.S.   National Institute of Standards and Technology/public domain
Been Here
Want to go
Added to list

About

From 1889 to 2019, the mass of a kilogram was defined by an object known as the international prototype kilogram (IPK) or, less formally, Le Grand K. A kilogram was a kilogram because of this object: a platinum alloy cylinder that sits in a vault on the outskirts of Paris.

Le Grand K is located in an environmentally monitored safe in a lower vault in the basement of the International Bureau of Weights and Measures in Sèvres, Paris. Housed under three bell jars, access to this precious cylinder requires three keys, operated independently, of which only two are kept in France.

Since its creation in 1889 (it was forged in London before being taken to France), the cylinder has only been removed from its secure housing once every 40 years. This rare appearance only occurs so that its mass can be checked against sister copies, which are housed in locations across the globe. It's a delicate process, as hydrocarbons on fingertips or moisture in the air could contaminate the original's pristine surface, potentially causing tiny changes in mass to the world's definition of a kilo.

But the original international prototype kilogram, by its very definition, could not change mass even if was subject to particulate change over time. It was, after all, the official kilogram, and therefore its mass was always one kilogram, even if it actually gains or loses mass. Its sister copies, however, are subject to measurable change in comparison to the original.

Over the past century, the trend for most sister copies has been to gain mass relative to the original by varying amounts, although these amounts are unimaginably tiny. On average, the gain is around 50 micrograms (millionths of a gram) over 100 years. It's possible, of course, that the original was losing mass relative to its copies, or that it's a combination of both. Either way, it's no great cause for concern for most of us, as the change in mass is roughly the weight of a fly's wing.

Still, for the men and women whose job it is to control and define these things, even the weight of a fly's wing is an unacceptable change. Something had to be done. A move was made to change how the kilogram is defined. And this is where it all gets very complicated.

Rather than rely on a platinum cylinder in a bell jar in Paris, eggheads in the world of measurements decided to anchor the future kilogram to Planck’s constant. This is a fixed quantity tied in with E=MC2 and quantum theory, specifying the amount of energy carried by a single particle of light, or photon. And that's just the most extremely simplified version.

All this was carried out under the watchful eye of the General Conference on Weights and Measures (CGPM) and an international task force known as the Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants (TGFC). Together, they came up with a value to redefine the kilogram in terms of the fundamental constants of nature.

Update 2019: Le Grand K is no longer used to define a kilogram.

Related Tags

Marvelous Maps And Measures Mathematics Science Instruments Of Science Government

Know Before You Go

The International Bureau of Weights and Measures is situated in the Parc de Saint-Cloud, at Sèvres, in the suburbs southwest of Paris. You can get there by taking Metro line 9 to Pont-de-Sèvres, then bus number 171, 169, 179 or 426 to the second stop, Parc de Saint-Cloud. Visiting hours are not published, so you should email in advance to see if you can arrange a visit. A replica of the International Prototype Kilogram is on display at the Cité des Sciences et de l’Industrie (Science and Industry Museum) in Paris.

Community Contributors

Added By

Tony Dunnell

Edited By

Martin, firekira, prvogelsang, graingert

  • Martin
  • firekira
  • prvogelsang
  • graingert

Published

August 29, 2018

Edit this listing

Make an Edit
Add Photos
Sources
  • https://www.theguardian.com/science/shortcuts/2017/jul/26/original-kilogram-le-grand-k-relegated-metric
  • https://www.scientificamerican.com/article/the-kilogram-may-be-redefined/
  • http://mentalfloss.com/article/31122/not-so-perfect-kilogram-and-why-metric-system-might-be-screwed
  • https://www.scmp.com/business/article/2071765/end-le-grand-k-lets-now-forget-importance-correct-measurement
  • https://www.sciencealert.com/researchers-now-have-a-better-measurement-for-defining-a-kilogram
  • http://news.bbc.co.uk/1/hi/sci/tech/7084099.stm
  • https://www.nist.gov/si-redefinition/kilogram-present
Le Grand K
Avenue du Pavillon de Breteuil
Saint-Cloud
France
48.829332, 2.220054
Visit Website
Get Directions

Nearby Places

Le Corbusier's Studio-Apartment

Boulogne-Billancourt, France

miles away

The Grande Lunette

Meudon, France

miles away

Villa La Roche by Le Corbusier

Paris, France

miles away

Explore the Destination Guide

Photo of France

France

Europe

Places 692
Stories 72

Nearby Places

Le Corbusier's Studio-Apartment

Boulogne-Billancourt, France

miles away

The Grande Lunette

Meudon, France

miles away

Villa La Roche by Le Corbusier

Paris, France

miles away

Explore the Destination Guide

Photo of France

France

Europe

Places 692
Stories 72

Related Stories and Lists

So Much Used to Depend on a Small Metal Cylinder in a Vault in France

objects of intrigue

By Jessica Leigh Hester

Related Places

  • NIST kilogram masses characteristic of the National Prototype Standard

    Gaithersburg, Maryland

    U.S. National Prototype Kilogram

    This golfball sized platinum iridium cylinder is the official starting point for all national weight calibrations

  • Uppsala, Sweden

    Anders Celsius's Thermometer

    The first thermometer made with the almost-universal temperature scale is backwards.

  • Paris, France

    The Last Original Standard Metre

    An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

  • The Liebniz Calculator.

    Hanover, Germany

    Leibniz's Stepped Reckoner

    An extraordinary mechanical calculator designed by the mathematician and philosopher Gottfried Leibniz.

  • Meyrin, Switzerland

    CERN

    Home of the Large Hadron Collider and the birthplace of the World Wide Web.

  • Glasgow, Scotland

    George Square Imperial Measurements

    A small collection of objects of a bygone era hide in plain view.

  • Leuven, Belgium

    Leuven Historical Measures

    Though they might seem like arbitrary scratches in the wall, these were once official measures of length.

  • Ladd Observatory

    Providence, Rhode Island

    Ladd Observatory

    A 132-year-old telescope continues to keep watch over the night skies above Providence.

Aerial image of Vietnam, displaying the picturesque rice terraces, characterized by their layered, verdant fields.
Atlas Obscura Membership

Become an Atlas Obscura Member


Join our community of curious explorers.

Become a Member

Get Our Email Newsletter

Follow Us

Facebook YouTube TikTok Instagram Pinterest RSS Feed

Get the app

Download the App
Download on the Apple App Store Get it on Google Play
  • All Places
  • Latest Places
  • Most Popular
  • Places to Eat
  • Random
  • Nearby
  • Add a Place
  • Stories
  • Food & Drink
  • Itineraries
  • Lists
  • Video
  • Podcast
  • Newsletters
  • All Trips
  • Family Trip
  • Food & Drink
  • History & Culture
  • Wildlife & Nature
  • FAQ
  • Membership
  • Feedback & Ideas
  • Community Guidelines
  • Product Blog
  • Unique Gifts
  • Work With Us
  • About
  • FAQ
  • Advertise With Us
  • Advertising Guidelines
  • Privacy Policy
  • Cookie Policy
  • Terms of Use
Atlas Obscura

© 2025 Atlas Obscura. All Rights Reserved.