Why Do They Keep Reweighing the Kilogram: A Deep Dive into Metrology
The kilogram, one of the seven base units of the International System of Units (SI), has been the subject of numerous redefinitions throughout its history. This article explores the reasons behind these reweighings and the ongoing efforts to define the kilogram in a more accurate and scientifically based manner.
The Historical Context
Until recently, the mass of the kilogram was defined by the International Prototype Kilogram (IPK), also known as the Grand K. This prototype, housed in France, was a cylinder of platinum-iridium alloy used as the standard reference for the kilogram. The kilogram was defined as being the mass of the Grand K. with a tolerance of plus or minus 1 part per million (ppm), equivalent to about 1 milligram (mg).
The Limitations of the Prototype
However, this definition has inherent limitations. Despite being kept in a highly controlled environment to protect against contamination, the Grand K and its copies have shown slight variations in mass over time. These variations, as small as a few micrograms (μg), can accumulate and become significant over decades. This phenomenon is known as drift.
The Recent Redefinition
To address these issues and ensure more accurate measurements, the International Committee for Weights and Measures (CIPM) decided to redefine the kilogram. On May 20, 2019, the new definition was introduced, based on the Planck constant (h). This new definition states that the kilogram is defined by the mass of a specific number of silicon-28 atoms (1/28 602 214 075 120 000 000 000 000 atoms).
Methods of Measurement
The new definition relies on various scientific methods, such as the Kibble balance (also called the Watt balance), which measures mass in terms of electrical and magnetic fields, or the direct counting of atoms. Another possibility is the creation of a silicon ball with a specific diameter (93.17 centimeters), where the volume is determined by the density of silicon-28.
Challenges and Future Milestones
While the new definition aims to provide a more stable and accurate standard, it introduces new challenges. The Kibble balance and atom counting methods each have their margins of error, which must be carefully measured and understood. Continuous monitoring and reassessment of these methods are crucial to ensure their accuracy and reliability.
Metrology and the Kilogram
Metrology, the science of measurement, is at the heart of these redefinitions. Ensuring the accuracy of the kilogram is crucial not only for scientific research but also for many practical applications in industry, commerce, and everyday life. The variability of the Grand K has led to a critical need for redefinition, and the new standard aims to provide a more stable and objective reference.
Further Reading and Resources
To delve deeper into this fascinating subject, you can watch a detailed video about the metrology of the kilogram. This video provides an excellent history lesson on the evolution of the kilogram and the challenges faced in defining it. Additionally, understanding the precise nature of measurements, such as weight vs. mass, can be crucial in various fields.
Conclusion: The ongoing efforts to redefine the kilogram highlight the importance of scientific precision and the need for continuous improvement in the field of metrology. As we move forward, the redefinition of the kilogram serves as a testament to humanity's commitment to accuracy and reliability in measurement.