Understanding the historical evolution of the kilogram is essential for anyone involved in science, technology, engineering, or mathematics.
This tutorial explores the history of the kilogram and recent redefinitions.
Origin of the Kilogram
Introduced during the French Revolution, the kilogram was adopted by France in 1795 as part of the metric system. This new system used universal natural properties for consistency. The kilogram was originally defined as the mass of one liter (1 cubic decimeter) of pure water at its freezing point (0°C).
However, using water as a practical standard posed difficulties because density can vary slightly due to impurities and temperature. Thus, scientists sought a more stable reference.
Today, kilograms (kg) are used widely to measure the mass of many things across various fields including everyday items like fruits, vegetables, meat, and grains as well as investments in precious metals like gold and silver.
The International Prototype Kilogram (IPK)
In 1879, scientists established the kilogram based on a physical artifact—the International Prototype Kilogram (IPK)—a cylindrical platinum-iridium alloy object. Officially adopted in 1889, the IPK served as the primary international standard for more than 130 years, housed securely in a vault at the International Bureau of Weights and Measures (BIPM) near Paris, France.
The IPK is a cylinder approximately 39 mm tall by 39 mm in diameter, composed of 90% platinum and 10% iridium. This alloy was chosen for its density, durability, and stability.
The IPK presented issues over the long term. It is susceptible to microscopic contamination and surface degradation as a physical object. Over time, this leads to slight variations in its mass that have potential implications for precision measurements globally.
Recognizing these limitations, scientists sought a more fundamental, universally constant definition based on fundamental physical constants.
Redefining the Kilogram with Fundamental Constants
On May 20, 2019, the kilogram was redefined based on the Planck constant, hh, a fundamental constant of quantum mechanics, linking the kilogram to the unchanging properties of nature rather than a physical artifact.
The exact value of the Planck constant was set at:
h=6.62607015×10−34 kg\cdotpm2/sh = 6.62607015 \times 10^{-34}\, \text{kg·m}^2\text{/s}
This redefinition leverages the relationship between mass and energy, expressed through quantum physics, ensuring long-term stability and reproducibility.
How the New Definition Works in Practice
The kilogram is now practically realized using instruments called Kibble balances (previously known as watt balances). The Kibble balance compares electrical power to mechanical power, leveraging electromagnetic forces and precise quantum electrical standards (the Josephson effect and quantum Hall effect) to determine mass accurately.
A Kibble balance measures the electromagnetic force necessary to counterbalance a given mass. Because the relationship between electrical power and the Planck constant is exact, the balance allows direct calibration of mass standards against fundamental physics.
Precision Engineering and the Kilogram
The redefinition significantly enhances precision measurement and engineering. Laboratories worldwide no longer need direct access to a single artifact to calibrate mass measurements, improving reproducibility, accuracy, and traceability in fields ranging from precious metals and engineering to aerospace and materials science.
Precision engineering benefits from this definition, as instruments and measurement tools can now be constructed with certainty, confidence, and universal reproducibility—crucial in research requiring accuracy at atomic or quantum levels.
Future Implications
With the kilogram’s redefinition, scientists can measure mass at extreme precision with consistency rooted firmly in mathematical constants. This ensures stability and accuracy and secures the kilogram’s future as an exact, universally reliable unit.