Measuring Mass: The Last Artefact – Just outside Paris, inside a hi-tech vault, and encased in three vacuum-sealed bell jars, rests a small metallic cylinder about the diameter of a golf ball. It may not look like much, but it is one of the most important objects on the planet.
It is the kilogramme, the base unit of mass in the International System of Units. This small hunk of metal is the object against which all others are measured. Yet over time, its mass has mysteriously eroded by the weight of an eyelash. A change that, unbeknownst to most, unleashed a crisis with potentially dire consequences. It affects nearly every aspect of our lives including the food we eat, the cars we drive, even the medicines we take.
Measuring Mass: The Last Artefact follows the ensuing high-stakes, two-year race to redefine the weight of the world, and tells the story of one of the most important objects on the planet.
Measuring Mass: The Last Artefact – A Brief History of the Kilogram
The kilogram is the base unit of mass in the International System of Units (SI) and has a long and fascinating history. It was first introduced as a unit of mass during the French Revolution and was later adopted as the global standard for mass measurement. In this article, we will explore the origins of the kilogram, its transition to a new definition, and the implications of this change for the scientific community.
The Original Kilogram: The International Prototype Kilogram (IPK)
The kilogram was originally defined as the mass of a platinum-iridium cylinder, known as the International Prototype Kilogram (IPK). This cylinder was manufactured in 1889 and was stored under controlled conditions in a vault at the International Bureau of Weights and Measures (BIPM) near Paris, France. For over a century, the mass of the IPK served as the global reference for the kilogram, with copies distributed to countries around the world for calibration purposes.
The Transition to the New Kilogram Definition
Two major efforts led the way to the redefinition of the kilogram: the Kibble balance and the Avogadro Project.
The Kibble Balance
The Kibble balance is a highly precise electromechanical weighing scale that measures mass by balancing the gravitational force acting on an object with the electromagnetic force produced by an electric current. The balance relies on the precise measurement of voltage and resistance, which can be traced back to fundamental constants of nature, such as the Planck constant.
The Avogadro Project – Measuring Mass
The Avogadro Project aimed to redefine the kilogram by counting the number of atoms in a silicon sphere of known mass. Using X-ray crystallography and other advanced techniques, researchers were able to determine the number of silicon atoms in the sphere with exceptional accuracy. This allowed them to calculate the mass of a single silicon atom, providing another path to the redefinition of the kilogram.
Why Redefining the Kilogram Was Necessary
The Problem with the International Prototype Kilogram
Despite the careful preservation of the IPK, its mass was found to be changing over time, albeit by minuscule amounts. This instability posed significant challenges for the scientific community, as it undermined the accuracy and reliability of mass measurements.
The Need for a More Stable and Universally Accessible Unit
The new kilogram definition, based on fundamental constants of nature, offers a stable and universally accessible unit of mass. This change was necessary to ensure the continued advancement of scientific research and the precision of measurements.
The New Definition of the Kilogram – Measuring Mass
The Planck Constant
The Planck constant (h) is a fundamental constant of nature that describes the relationship between energy and frequency. The redefinition of the kilogram is based on a fixed value of the Planck constant, which is determined through experiments using the Kibble balance and the Avogadro Project.
How the Planck Constant was Determined
Researchers used the Kibble balance to measure the Planck constant with extraordinary precision by comparing the mechanical power needed to balance a known mass with the electrical power required to generate an equal force. Meanwhile, the Avogadro Project provided an alternative method of determining the Planck constant by counting the number of atoms in a silicon sphere. These two independent methods converged on a consistent value, providing a solid foundation for the redefinition of the kilogram.
Implications of the New Kilogram Definition
Increased Precision in Scientific Measurements
The new definition of the kilogram, based on the Planck constant, offers greater precision and stability in mass measurements. This increased accuracy is essential for many areas of scientific research, including fields like particle physics, metrology, and materials science.
Accessible and Reproducible Measurements
The new kilogram definition allows for the mass unit to be determined independently by different laboratories using various experimental methods. This accessibility enables scientists worldwide to reproduce the measurements, promoting international collaboration and ensuring that the kilogram remains a universally accepted unit of mass.
Conclusion – Measuring Mass
The transition from the International Prototype Kilogram to a new definition based on the Planck constant marked a significant milestone in the history of measurement. The new kilogram offers greater precision, stability, and accessibility, benefiting the scientific community and ensuring the continued advancement of research and technology.
Frequently Asked Questions
What is the kilogram?
The kilogram is the base unit of mass in the International System of Units (SI) and is defined using the fixed value of the Planck constant.
Why was the kilogram redefined?
The kilogram was redefined to address the instability in the mass of the International Prototype Kilogram and to provide a more stable and universally accessible unit of mass based on fundamental constants of nature.
What is the Planck constant?
The Planck constant is a fundamental constant of nature that relates the energy of a photon to its frequency. It plays a crucial role in the new definition of the kilogram.
What are the Kibble balance and the Avogadro Project?
The Kibble balance is an electromechanical weighing scale that measures mass by comparing gravitational and electromagnetic forces. The Avogadro Project determined the mass of a silicon atom by counting the number of atoms in a silicon sphere. Both methods were used to establish the new definition of the kilogram.
How does the new kilogram definition impact scientific research?
The new kilogram definition offers increased precision, stability, and accessibility, which are essential for the advancement of scientific research and the accuracy of mass measurements in various fields.
