The kilogram is used worldwide to measure everyday items such as food, body weight, as well as commodities like silver and gold.
The definition of a kilogram is part of the International System of Units (SI), which is widely used globally in science, industry, and everyday life.
In precious metals, a kilogram bar is a common unit used by both investors and large institutions like central banks.
Scientific Definition of a Kilogram
Scientifically, the kilogram is defined by taking the fixed numerical value of the Planck constant ℎ as 6.62607015×10−346.62607015 × 10^{-34}6.62607015×10−34 joule seconds (J·s), with the meter and the second being defined in terms of the speed of light and the cesium frequency, respectively. This definition links the kilogram to fundamental physical constants, ensuring long-term stability and precision.
In scientific research and various industries, the kilogram is crucial for precise measurements is widely used in calibrations and standards for manufacturing processes, pharmaceuticals, and materials science.
History of the Kilogram
The history of the kilogram is deeply rooted in the development of the metric system during the French Revolution and has evolved significantly over time.
Origins of the Kilogram
The kilogram was first defined in 1795 as the mass of one liter (10^-3 cubic meters) of water at the freezing point. This was part of the metric system developed during the French Revolution in order to create a uniform system of measurements based on natural constants.
In 1799, the definition was refined, and a platinum artifact known as the “Kilogramme des Archives” was created as a unit of mass and became the basis for the kilogram until the late 19th century.
The International Prototype Kilogram (IPK)
The General Conference on Weights and Measures (CGPM) established the International Prototype Kilogram (IPK) in 1889, made of a platinum-iridium alloy. This artifact was stored at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.
The IPK was the primary standard for the kilogram for over a century, with numerous copies made and distributed to various countries for calibration purposes.
Issues with the IPK and Re-definition Efforts
Over time, it became apparent that the mass of the IPK and its copies could change due to surface contamination or material degradation, leading to inconsistencies.
This prompted the scientific community to seek a more stable and precise definition based on fundamental constants of nature.
Current Kilogram Definition
On May 20, 2019, the kilogram was redefined based on the Planck constant (h), a fundamental constant of nature. This marked a significant shift from using a physical object, to a definition based on a fundamental property of physics.
The new definition uses the fixed numerical value of the Planck constant, h=6.62607015×10−34Jsh = 6.62607015 \times 10^{-34} \text{Js}h=6.62607015×10−34Js, along with definitions of the meter and the second, to establish the kilogram. This approach ensures long-term stability and precision.
This redefinition also aligns with the broader goal of basing all SI units on fundamental constants rather than physical artifacts.
Summary
The kilogram’s history reflects the evolution of metrology from practical measurements to highly precise and stable definitions based on the fundamental constants of nature. The shift from the IPK to a constant-based definition represents a major milestone in scientific measurement, ensuring greater accuracy and consistency in scientific research and industry.
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