The Changing Definition of the Kilogram: From Physical Artifact to Fundamental Constant
From the humble 1668 proposal by Bishop John Wilkins to the cutting-edge Kibble balance, the definition of the kilogram has undergone a profound transformation. Historically, the kilogram was defined by a physical object, the International Prototype of the Kilogram (IPK). However, this characterization has inherent limitations. With the advent of the Planck constant as a fundamental constant, the definition of the kilogram has shifted to a more stable and precise standard, rooted in the natural world.
Historical Context: The Kilogram as a Physical Artifact
In 1668, mathematician Bishop John Wilkins proposed a coherent system of measurement. He starting with the second pendulum, defining the length as a meter. A cube with the side length of one-tenth of that meter (hence 10 cm) was filled with pure water, and the mass of that cube became the kilogram.
For a visual demonstration of this concept, please refer to the following video: [Link to Video].
Historically, the kilogram was defined by a specific platinum-iridium cylinder, carefully stored in a vault in France. This cylinder, known as the International Prototype of the Kilogram, had a defined mass. However, this physical standard was not ideal due to potential changes in mass due to surface contamination or material degradation over time.
Transition to a Fundamental Constant Definition
As of May 20, 2019, the kilogram was redefined in terms of the Planck constant, a fundamental constant of physics. The new definition uses a fixed value of the Planck constant (6.62607015 × 10?34 J?s) and is linked to the International System of Units (SI) via the definitions of other units such as the meter and the second.
This change ensures that the kilogram is more stable and reproducible, as it relies on universal constants rather than a physical object that can change. It also allows for greater precision in scientific measurements and applications, benefiting fields such as metrology, physics, and engineering.
Technical Details: Kibble Balance and Planck Constant
Before the kilogram could be redefined in terms of the Planck constant, the technology for precisely measuring mass in terms of these constants had to be developed. The Kibble balance, also known as the watt balance, is a key instrument that overcame this hurdle.
The Kibble balance measures mass indirectly by balancing mechanical power against electrical power. It utilizes the Planck constant to define the kilogram, thereby ensuring a fixed value for the unit. This balance is a critical component in the future of precision metrology and the definition of base units in the International System of Units (SI).
The redefinition of the kilogram in terms of the Planck constant was not possible earlier due to the lack of precise measurement technology. The Kibble balance, however, has made this transition feasible, highlighting the importance of advancements in technology in scientific standardization.
To understand the Kibble balance and its application in defining the kilogram, please explore the Wikipedia article on the Kibble balance.
With the redefinition of the kilogram, the world of metrology and scientific measurements has entered a new era. The kilogram, now defined by a fundamental constant, offers unprecedented precision and stability, ensuring that scientific and technological advancements continue to flourish.