Kilogram
In 1795, the French National Convention defined a new unit of mass based on water. They declared that one gram would equal the mass of one cubic centimetre of water at the melting point of ice. This decision emerged from the chaos of the French Revolution when leaders sought to replace old royal measures with rational standards. The definition relied on precise measurements of temperature and volume. Scientists determined that water reaches its maximum density near four degrees Celsius. By 1799, they manufactured the Kilogramme des Archives as a physical prototype. This platinum cylinder replaced the theoretical water-based definition for practical use. The archive kilogram weighed exactly one thousand grams. It served as the standard for France until international agreements required a global solution.
The Metre Convention signed in 1875 led to the creation of the International Prototype of the Kilogram. A platinum-iridium cylinder known as the IPK became the official standard in 1889. This object remained the definition of mass for over 130 years. Scientists stored the original cylinder in a vault near Paris alongside copies distributed to member nations. Over time, these replicas began to diverge from the master cylinder. Measurements showed differences of approximately 50 micrograms since their manufacture late in the 19th century. Such drift created uncertainty about whether the standard itself was changing or if the copies were losing mass. The discrepancy grew large enough to threaten scientific accuracy. Researchers realized that relying on a single metal artifact posed risks for future precision work. The problem demanded a new approach before the divergence could cause widespread errors.
On the 20th of May 2019, the General Conference on Weights and Measures adopted a new definition. The kilogram no longer depended on a physical object but instead relied on fundamental constants of nature. Specifically, scientists fixed the value of the Planck constant to define mass. This change linked the unit to the speed of light and the hyperfine transition frequency of caesium-133 atoms. The decision followed decades of research into measurement technologies capable of realizing this new standard. The International Committee for Weights and Measures approved the revision in November 2018 after confirming data robustness. Laboratories now use devices like the Kibble balance to calibrate secondary standards without needing the original prototype. Any apparatus capable of delineating the kilogram through these constants can serve as a primary standard. The shift ensured that the unit remains stable regardless of environmental changes affecting physical objects.
The word kilogram combines the Greek prefix kilo meaning one thousand with the Latin term gramma. French law first wrote the term into official records during the Decree of 18 Germinal in 1795. It replaced earlier terms used by the National Convention two years prior. English speakers adopted the spelling from Great Britain in 1795 while the United States embraced it shortly after. The term became unique among SI base units because it includes an SI prefix within its name. Other units add prefixes to the gram rather than the kilogram itself. For example, scientists write milligram instead of microkilogram. The abbreviation kilo gained popularity in the 19th century but lacks formal status in scientific writing. The Canadian government Termium Plus system describes it as common informal usage. The United States Congress revoked legal permission for kilo in 1990 despite allowing it since 1866.
Modern metrology laboratories utilize specialized equipment to realize the new definition of mass. The Kibble balance measures electrical quantities to determine weight and convert them into mass values. This device originally measured the Planck constant relative to the IPK before becoming a calibration tool. Another approach involves the Avogadro project which counts atoms in silicon spheres to establish precise mass standards. Both methods require converting weight measurements into mass using gravimetry techniques. Scientists must measure the strength of gravity in their specific laboratory locations with extreme precision. These technologies allow reproducible production of kilogram-mass prototypes on demand if needed. The process depends on material properties traceable directly to physical constants. No single artifact defines the unit anymore. Instead, any apparatus meeting strict accuracy requirements can serve as a primary standard globally.
Medical professionals face serious risks when confusing milligrams and micrograms in prescriptions. The abbreviation mcg replaced the symbol μg in US healthcare settings due to safety mandates from the Joint Commission on Accreditation of Healthcare Organizations. UK guidelines require writing out micrograms and nanograms fully without abbreviations. Retail food trade in Italy frequently uses the hectogram or ettogrammo for transactions involving coffee or meat. Central European markets still employ the abbreviation dkg for 10 grams when selling cheese. The term megagram rarely appears outside technical fields where consistency matters most. Most people prefer the word tonne instead of megagram for large masses. The tonne gained adoption by the CIPM in 1879 as a non-SI unit accepted for use with SI. Regional variations persist despite global standardization efforts. These differences highlight how local customs influence scientific communication even within unified systems.
Common questions
When was the kilogram first defined by the French National Convention?
The French National Convention defined the kilogram in 1795. They declared that one gram would equal the mass of one cubic centimetre of water at the melting point of ice.
What happened to the International Prototype of the Kilogram after 1889?
Scientists stored the original cylinder in a vault near Paris alongside copies distributed to member nations. Measurements showed differences of approximately 50 micrograms since their manufacture late in the 19th century.
On what date did the General Conference on Weights and Measures adopt the new definition of the kilogram?
On the 20th of May 2019, the General Conference on Weights and Measures adopted a new definition. The kilogram no longer depended on a physical object but instead relied on fundamental constants of nature.
Why does the word kilogram include an SI prefix within its name?
The term became unique among SI base units because it includes an SI prefix within its name. Other units add prefixes to the gram rather than the kilogram itself.
How do modern laboratories realize the new definition of mass without using the original prototype?
Laboratories now use devices like the Kibble balance to calibrate secondary standards without needing the original prototype. Any apparatus capable of delineating the kilogram through these constants can serve as a primary standard.