Metre Convention facts for kids
The Metre Convention is an important treaty that created the International Bureau of Weights and Measures (BIPM). The BIPM is a group of governments working together. Their job is to make sure that measurement systems are the same all over the world. This treaty was first signed in 1875 by 17 countries.
These countries agreed to share the costs of running a special laboratory. This lab is in a neutral place, meaning it doesn't belong to any one country. In this lab, they keep the official international metre and kilogram standards. Countries that are part of the treaty regularly compare their own national metres and kilograms to these international standards. They also meet often to talk about new ways to measure things.
In 1921, the treaty was updated. It now included measurements for electrical and all other physical things. This allowed the BIPM to create the International System of Units (SI). SI is a system of measurement units used in science and engineering. It defines units in a clear and logical way. The rules for SI also made it standard how physical quantities are written down. By 2014, the BIPM had 55 member countries.
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Why We Needed the Metre Convention
The metric system started in France in the 1790s. The idea was to make the metre exactly one ten-millionth of the distance from the North Pole to the equator. Between 1791 and 1798, two French scientists, Delambre and Méchain, measured the distance between Dunkirk and Barcelona. They used these measurements to create a metal bar. This bar was meant to be the official standard for the metre. It was called the metre des archives.
However, by the 1860s, scientists found a problem. The metre des archives was actually 0.03% shorter than it was supposed to be.
Big events like The Great Exhibition in London (1851) and the Paris International Exhibition (1867) showed a clear need. Different countries needed to use the same units of measurement. In the 1860s, scientists from outside France debated. Should they keep using the slightly wrong metre des archives? Or should they create a "new metre" that was perfectly accurate?
Prussia took the lead and organized meetings in Berlin to discuss this. France did not join these meetings. Before anything could be decided, the Franco-Prussian War started. Prussia won the war, and the German Empire was formed. In 1875, France invited representatives to Paris to discuss the matter. This helped France gain a diplomatic win over Prussia and the German Empire.
What the Treaty Decided
Seventeen countries signed the treaty in Paris on May 20, 1875. The United Kingdom and the Netherlands attended but did not sign right away. The United Kingdom later signed the treaty in 1884.
The meeting agreed on several key points:
- Three organizations would be created to manage the treaty.
- Exact copies of the metre and kilogram would be made. One copy of each would be chosen as the international master copy. Other copies would be given to member countries.
- The master copies of the metre and kilogram would be kept safe in the laboratory.
- This laboratory would be on neutral ground.
- Member countries would regularly compare their copies with the master copies.
- The organizations would encourage everyone to use the metric system.
The International Bureau of Weights and Measures (BIPM)
The main organization set up by the treaty is the International Bureau of Weights and Measures. Its French name is Bureau international des poids et mesures, which is why its short name is BIPM. The BIPM's main office is at the Pavillon de Breteuil, in Sèvres, France, near Paris. About 70 people work there. In 2013, its budget was over €10 million (about $12 million).
The treaty created three main organizations:
- The International Bureau of Weights and Measures (BIPM).
- The General Conference on Weights and Measures (CGPM).
- The International Committee for Weights and Measures (CIPM).
The BIPM is in charge of keeping the international prototype kilogram safe.
The official language for these organizations is French. Until 1988, all meeting reports were only in French. Now, they use both English and French. The short names (like BIPM) come from the French words. The BIPM talks to France through the French Foreign Ministry. For other countries, they communicate through that country's embassy in Paris.
The General Conference on Weights and Measures (CGPM)
The General Conference on Weights and Measures (CGPM) was created by the Metre Convention. It tells the BIPM what to do and approves its budget. Its French name is Conférence générale des poids et mesures.
The CGPM meets every four to six years. The first meeting was in 1889, and the 24th was in 2011. Each member country gets one vote. France is represented by its foreign minister and the president of the French Academy. The French Foreign Minister leads the start and end of each CGPM meeting. The president of the French Academy leads other parts of the meetings.
The CGPM's jobs include:
- Approving the BIPM's budget.
- Agreeing on how much member countries should pay.
- Choosing members for the CIPM.
- Approving ideas from the CIPM.
The International Committee for Weights and Measures (CIPM)
The International Committee for Weights and Measures (CIPM) was also set up by the Metre Convention. It advises the CGPM and oversees the BIPM. Its French name is Comité international des poids et mesures.
The CIPM has 18 members. These are well-known experts in measurement, chosen by the CGPM. They must all come from different countries, and one must be French. They meet once or twice a year in Paris. The CIPM gets help from ten special groups called Consultative Committees. Nine of these committees focus on specific types of measurement. These include Electricity and Magnetism, Length, Light Measurement, Temperature Measurement, and Time and Frequency. The CIPM chooses people for these committees.
The tenth committee is the Consultative Committee for Units (CCU). Its leader is chosen by the CIPM. Other members come from groups like the International Organization for Standardization (ISO) and the National Institute of Standards and Technology (NIST). The CCU makes sure all the definitions in SI are correct. It also writes the official SI standard, called the SI Brochure.
The Pavillon de Breteuil
The BIPM's offices and labs are located at the Pavillon de Breteuil in Sèvres, France, close to Paris.
The BIPM is responsible for looking after the International Prototype Kilogram (IPK) for the CGPM. It also provides laboratory services for the CGPM and CIPM. Member countries use the BIPM labs to check their own copies of the kilogram against the IPK. The BIPM offices also serve as the main headquarters for the CGPM and CIPM.
What the Metre Convention Does
In 1875, the convention only dealt with making the kilogram and metre standard. The convention was updated in 1921 to include all kinds of physical measurements. In 1933, the CGPM approved efforts to add electrical and light measurement units to the metric system.
Creating the SI System
In the late 1800s and early 1900s, many new units of measure were created using the metric system. However, units used for static electricity, magnetism, and electrical power were not always compatible. Also, using standard gravity in definitions for force and pressure led to too many unnecessary units.
After World War II, the International Union of Pure and Applied Physics (IUPAP) and the French Government asked the BIPM to fix this. In 1948, the 9th CGPM asked the CIPM to study the units of measurement used by member countries. The CIPM reported back, and in 1954, the 10th CGPM decided to create a new system of units. This new system would have six basic units: the metre, kilogram, second, ampere, degree Kelvin (later called kelvin), and candela.
This new system was officially published in 1960. The 11th CGPM named it the International System of Units, or "SI" for short. This short name comes from its French name, [Le Système international d'unités] Error: {{Lang}}: text has italic markup (help). The BIPM called SI "the modern metric system." Since 1960, the BIPM has made many improvements to SI. This includes finding new ways to make very accurate measurements and making small changes to unit definitions.
Keeping Time Coordinated
Atomic clocks allow for very precise time-keeping. The Earth is actually slowing down. Millions of years ago, when dinosaurs lived, the Earth rotated once every 23 hours. Now, we can measure tiny changes in the Earth's rotation caused by things like volcanoes and tsunamis. The International Earth Rotation and Reference Systems Service (IERS) does this. The IERS suggests adding leap seconds when needed to keep our clocks in sync with the Earth. Under the Metre Convention, the BIPM's job is to coordinate over 200 atomic clocks around the world. These clocks are in 50 different national laboratories.
Mutual Recognition Agreement (MRA)
Many measuring devices need to be checked and adjusted regularly. Think about petrol pumps, weighing scales, or complex medical machines like MRI scanners. When a device is checked, a certificate is given. The device used for checking must also be checked itself. These checks are often done in national measurement laboratories. If a check done in one country isn't accepted in another, it can cause legal problems. The MRA helps make sure these checks are recognized everywhere.
Who Are the Members?
Some countries are full members of the CGPM. They get to vote at meetings and have their own copy of the prototype kilogram. They also take part in the CIPM MRA program.
Other countries are associate members. They can join the CIPM MRA program and speak at CGPM meetings. However, they cannot vote, and they do not have their own copies of the prototype kilogram.
Member Countries
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Associate Member Countries
In October 1999, the CGPM created a new category called "associate" members. This is for countries that are not yet full members of the BIPM and for economic unions.
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International Organizations
The following international organizations have signed the CIPM MRA:
- International Atomic Energy Agency (IAEA), Vienna, Austria (1999)
- Institute for Reference Materials and Measurements (IRMM), Geel, Belgium (1999)
- World Meteorological Organization (WMO), Geneva, Switzerland (2010)
- European Space Agency (ESA), Paris, France (2012)
Former Member Countries
Cameroon (1970–2012) 
North Korea (1982–2012) 
Dominican Republic (1954-2014) 
Yemen (2014-2018)
Images for kids
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This old picture from 1800 shows the new decimal units that became law in France.
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A view of the Pavillon de Breteuil, where the BIPM is located.
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This is the logo used by labs that are approved under the CIPM MRA program.
See also
In Spanish: Convención del Metro para niños
