Scientists during the French Revolution working to establish standardized measurements, including the kilogram

The kilogram, the fundamental unit of mass in the International System of Units (SI), has a rich history that reflects humanity's quest for precision in measurement. Like the meter, its story begins during the French Revolution, when the need for standardized, universal units became paramount.

Before the metric system, weights and measures varied widely between regions and countries, creating significant challenges for trade, science, and everyday life. The inconsistency of units was seen as a relic of the feudal past, and revolutionary France sought to replace this chaotic system with one based on reason and nature.

Origins in Water

In 1793, the French National Convention introduced the "grave" as a precursor to the kilogram, defined as the mass of one liter (cubic decimeter) of water. By 1795, the gram was provisionally defined as the mass of one cubic centimeter of water at the melting point of ice, with the kilogram being 1000 grams.

This definition tied the unit of mass directly to the unit of length (the meter) and to a substance universally available—water. It embodied the revolutionary ideals of creating a measurement system based on nature rather than arbitrary standards.

The First Physical Standard

In 1799, a platinum artifact called the Kilogramme des Archives was manufactured to represent this mass. It was designed to have a mass equal to one cubic decimeter of water at its maximum density (approximately 4°C). This physical embodiment of the kilogram served as the standard for the next 90 years.

The use of platinum was significant—its high density, resistance to oxidation, and relative stability made it ideal for a mass standard. However, pure platinum is relatively soft, which would lead to problems in the long term.

The International Prototype Kilogram

A major development came with the signing of the Metre Convention in 1875, which established the International Bureau of Weights and Measures (BIPM) to maintain international standards. This led to the creation of the International Prototype Kilogram (IPK) in 1879.

The IPK, often called "Le Grand K," was a cylinder made of 90% platinum and 10% iridium, with both height and diameter of 39 millimeters. The addition of iridium significantly improved upon the all-platinum Kilogramme des Archives by increasing hardness while maintaining platinum's desirable properties. In 1889, the IPK was formally adopted as the definition of the kilogram.

This remarkable artifact was stored in a vault at the BIPM headquarters in Sèvres, near Paris, protected by three independently controlled keys and housed under two nested glass bell jars. The IPK was rarely removed from its secure environment—only occasionally for comparison with its official copies.

Nations around the world received official copies of the IPK to serve as their national standards. In the United States, for example, prototype K20 served as the primary national standard. These national prototypes were periodically compared to the IPK, creating a chain of measurement traceability that extended throughout the world.

The Problem of Drift

Despite the care taken in its preservation, evidence accumulated over time that the mass of the IPK and its replicas was changing. By the late 20th century, measurements showed that the IPK had diverged from its official copies by approximately 50 micrograms since their creation—a tiny but scientifically significant amount.

This drift created a fundamental problem: if the kilogram was defined as the mass of the IPK, and that mass was changing, what did it mean for the definition of the kilogram? Moreover, if the IPK were damaged or destroyed, the entire system of mass measurement would be compromised.

This situation was particularly problematic because the kilogram underpinned many other units in the SI system. By 2018, it was the only SI base unit still defined by a physical artifact rather than a fundamental constant of nature.

The Quantum Revolution

Scientists recognized the need to redefine the kilogram in terms of invariant constants of nature, just as had been done with the meter (defined in terms of the speed of light) and the second (defined in terms of atomic transitions).

After decades of research, the International Committee for Weights and Measures (CIPM) approved a historic revision in November 2018. The kilogram would now be defined by fixing the numerical value of the Planck constant (h) to exactly 6.62607015 × 10^-34 when expressed in the unit J·s (joule-second).

This new definition, which took effect on May 20, 2019, effectively defines the kilogram in terms of the second and the meter. It creates a standard that is universal and invariant—one that could theoretically be reproduced by any advanced civilization anywhere in the universe.

The practical realization of this definition typically involves sophisticated instruments like the Kibble balance, which can measure mass in terms of electromagnetic force, ultimately traceable to the Planck constant.

Legacy and Significance

The evolution of the kilogram from a measure based on water to one defined by quantum physics reflects the broader development of science and metrology. Each step in this journey has brought greater precision, universality, and stability to our measurement systems.

The IPK, though no longer the defining standard, remains an important historical artifact. It represents a crucial chapter in humanity's quest to measure and understand the physical world. Its successor—a definition based on fundamental constants—ensures that the kilogram will remain stable and accessible for generations to come.

This redefinition completed the transformation of the SI system to one entirely based on fundamental constants of nature rather than physical artifacts. It represents one of the most significant developments in the history of measurement science, ensuring that our units of measure are truly universal and invariant.

The story of the kilogram reminds us that even the most basic concepts in science—like how we measure mass—continue to evolve as our understanding deepens and our technology advances. From revolutionary France to the quantum age, the kilogram's journey mirrors humanity's relentless pursuit of precision and understanding.