Friday, May 24, 2019

New Era of Measurement: Kilogram Now Defined for All-Time

Replica of the International Prototype Kilogram (IPK) on display at the Museum of Science and Industry (Cité des Science et de l’Industrie) in Paris, France. The actual IPK served as the international standard for the kilogram from 1889 until the beginning of this week.
(Image Sources:, By Japs 88 - Own work, CC BY-SA 3.0,

By Glenn A. Walsh
Reporting for SpaceWatchtower

This week, weight measure moved from the 19th century to the 21st century. The official international measurement of weight or mass, the kilogram (kg), was redefined from a physical prototype to a fundamental physical property of the Universe.

The kilogram is the measurement of mass or weight, originally part of the Metric System now known as the International System of Units (SI). In the United States which uses Imperial Units, 1 pound is defined as 0.45359237 kilograms.

On World Metrology Day, Monday, 2019 May 20, scientists implemented the final conversion of a measure of the SI from a physical artifact to a mathematical equation of a universal constant.

From 1889 until this week, the kilogram was defined as the mass of the International Prototype Kilogram (IPK), a golf-ball sized object stored in a vault with six precise replicas at the International Bureau of Weights and Measures [or in French: Bureau International des Poids et Mesures (BIPM)], located in Saint-Cloud, France on the outskirts of Paris. Two additional precise replicas exist at the United States National Institute of Standards and Technology in Gaithersburg, Maryland, just outside of Washington DC. And, several other countries around the world also possess precise replicas.

As a measurement standard for each country, these replicas have been needed for science experiments that require very precise weight measurements and for international trade in items restricted by weight, including radioactive materials.

Manufactured as a right-circular cylinder, the IPK is composed of a platinum alloy known as "Pt 10Ir", which is 90% platinum and 10% iridium (by mass). The addition of 10% iridium greatly improved the hardness from the previous, all-plantinum kilogram definition artifact: the Kilogram of the Archives (Kilogramme des Archives), produced as a prototype in 1799. In 1875, when it was determined to make a new kilogram definition artifact, the Kilogram of the Archives was used to derive the IPK.

The decision to modernize the definition of the kilogram came last November 16 in Versailles, France, as scientists unanimously approved the change at a meeting of the General Conference on Weights and Measures. However, this decision was many years in-the-making, as scientists deliberated on how to solve a problem with the International Prototype Kilogram.

As an international standard, the International Prototype Kilogram (also known to scientists as the “Big K” or “Le Grand K") was meant to be unchanging, to ensure standardization throughout the international system of the measurement of mass or weight. But, with a physical artifact, this was not possible.

As any human-made object is imperfect, so was the International Prototype Kilogram. The IPK has lost 50 micrograms (about the weight of an eyelash) since it was created in 1889. And, replicas in each country, meant to be the standard for each country based on the IPK in Paris, may have lost more, less, or no micrograms. Hence, the IPK could no longer be a precise standard, nor could any of the replicas.

So, beginning this-past Monday, the kilogram is now defined by a concept from Quantum Mechanics: the Planck Constant, derived by German theoretical physicist Max Planck in 1900. Max Planck received the 1918 Nobel Prize in Physics "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta".

The new definition of the kilogram is now determined to be the Planck constant, as defined by the ISO standard, set to 6.62607015×10−34 J⋅s exactly. Ironically, the old kilogram (International Prototype Kilogram) was used to measure the value of the Planck Constant.

So, now the kilogram is defined by an unchanging standard “for all times, for all people.”

The original 1889 International Prototype Kilogram will now be displayed in a museum, instead of being securely hidden-away in a vault.

The kilogram was not the only SI standard redefined on May 20. New definitions for electric current, temperature, and the amount of a substance also changed. As with the kilogram, all of these new definitions are also related to constants of the physical Universe.

Internet Links to Additional Information ---

Kilogram: Link >>>

International Prototype Kilogram:
Link >>>

Planck Constant: Link >>>

International Bureau of Weights and Measures:
Link >>>

U.S. National Institute of Standards and Technology (previously, National Bureau of Standards):
Link 1 >>>
Link 2 >>>

Related Blog Post ---

"Official Kilogram Standard Contaminated: Experiments Could be Affected.

The Kilogram Has Gained Weight." 2013 Jan. 6.

Link >>>

Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
              Friday, 2019 May 24.

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Glenn A. Walsh, Informal Science Educator & Communicator: >
Electronic Mail: < >
Project Director, Friends of the Zeiss: < >
SpaceWatchtower Editor / Author: < >
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
  < >
* Adler Planetarium, Chicago:
  < >
* Astronomer, Educator, Optician John A. Brashear:
  < >
* Andrew Carnegie & Carnegie Libraries:
  < >


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