The Evolution of Kilogram: From Prototype to Fundamental Constant
The kilogram, long a standard unit of mass, underwent a significant transformation on May 20, 2019. This change, part of a broader effort to redefine the International System of Units (SI), marked a shift from a physical object to a fundamental constant of nature.
From the International Prototype to the Planck Constant
Prior to 2019, the kilogram was based on a physical prototype called the International Prototype Kilogram (IPK). This prototype, a cylinder made of platinum-iridium, was stored in France and served as the standard against which all other kilograms were measured. However, this reliance on a physical object led to concerns about stability and reproducibility due to its potential to change over time from wear and contamination.
Defining Kilogram by the Planck Constant
The new definition of the kilogram is based on the Planck constant (h), one of the fundamental constants of nature. This change aligns the kilogram with other SI units, all now defined in terms of constants like the speed of light or the quantum of electric charge. The value of the Planck constant is approximately 6.62607015 × 10-34 Js.
The Role of the Kibble Balance
The Kibble balance, or watt balance, is a key instrument in this redefinition. This device measures mass by balancing gravitational forces against electromagnetic forces, effectively linking mass to electrical measurements. By using such a balance, the definition of mass is no longer tied to a physical artifact but to the fundamental laws of physics.
Impact on Science, Industry, and Commerce
The shift from a physical prototype to a fundamental constant has far-reaching implications. It enhances the precision and stability of mass measurements, which are crucial in various fields. In science, accurate mass measurements are essential for research in chemistry, physics, and beyond. In industry, precise measurements ensure the quality control of products, and in commerce, they ensure fair trade and accurate billing.
Education and Adoption
India, like many other countries, has taken steps to incorporate this change into its education system. Schools and technical institutes have been advised to include the change in their syllabi. The National Physical Laboratory (NPL), which is the custodian of the fundamental units of measurement, has recommended to the National Council of Educational Research and Training (NCERT), the All India Council for Technical Education, and other educational institutions.
A Pragmatic View: Defining Kilogram by Water
Some argue that the kilogram should remain defined as the mass of 1 liter of distilled water (pure H2O) due to its practicality. One liter of water at a standard temperature and pressure (STP) has a mass of 1 kilogram, making it a simple and intuitive standard. However, the shift to a fundamental constant offers greater precision and scientific rigor, ensuring that the definition of the kilogram remains consistent over time.
While the traditional definition based on water is pragmatic, the new definition based on the Planck constant ensures that the kilogram remains a standard of precision and stability, aligning it with the advancements in modern science and technology.