International Prototype of the Kilogram

International Prototype of the Kilogram
BIPM · CC BY-SA 3.0 igo · source
Name	International Prototype of the Kilogram
Caption	A replica of the International Prototype of the Kilogram.
Unit	Kilogram
Quantity	Mass
Established	1889
Replaced	2019

International Prototype of the Kilogram. For over a century, this object served as the definitive standard for the SI base unit of mass. Housed under strict conditions at the International Bureau of Weights and Measures near Paris, it was the physical artifact upon which all metric system mass measurements were ultimately based. Its eventual replacement by a definition based on fundamental constants marked a pivotal moment in metrology.

History and Development

The need for a universal mass standard emerged from the French Revolution and the subsequent creation of the metric system. The original standard, the Kilogram of the Archives, was crafted in Paris in 1799. Following the Metre Convention of 1875, which established the International Bureau of Weights and Measures, a new, more precise artifact was commissioned. British metallurgy firm Johnson Matthey produced a cylinder of platinum-iridium alloy in the 1870s. After meticulous polishing and adjustment at the Observatory of Paris, it was formally adopted as the International Prototype of the Kilogram by the General Conference on Weights and Measures in 1889.

Physical Characteristics

The prototype is a cylinder with a height equal to its diameter, measuring approximately 39 millimeters in each dimension. It is composed of an alloy of 90% platinum and 10% iridium, chosen for its exceptional hardness, density, and resistance to oxidation. Its edges are slightly chamfered to minimize wear. The artifact is stored within a nested arrangement of three sealed bell jars under a controlled atmosphere at the Pavillon de Breteuil in Sèvres. This environment protects it from contaminants and minimizes surface interactions with the air.

Role as the Primary Mass Standard

As the sole defining artifact for the kilogram, the prototype was the cornerstone of global mass metrology. Official copies, known as national prototypes, were distributed to member states of the Metre Convention, such as the National Institute of Standards and Technology in the United States and the National Physical Laboratory (United Kingdom). These copies were periodically returned to the International Bureau of Weights and Measures for calibration against the primary standard. This hierarchical system ensured worldwide uniformity in measurements critical to international trade, pharmaceuticals, and scientific research.

Limitations and Instability

Despite its careful preservation, the artifact-based definition had inherent flaws. As a physical object, it was susceptible to change from surface contamination, such as the adsorption of atmospheric hydrocarbons or the loss of material through cleaning. Comparative measurements conducted over decades, including those by physicist Peter Becker, indicated that the masses of the various prototypes were diverging from each other at the microgram level. This instability was unacceptable for modern precision science, especially in fields like quantum physics and cosmology, which require constants anchored to immutable properties of nature.

Redefinition and Retirement

The drive to redefine the SI base units in terms of fundamental constants culminated in a landmark decision by the General Conference on Weights and Measures in 2018. Effective on World Metrology Day 2019, the kilogram was redefined by fixing the numerical value of the Planck constant. This new definition is realized experimentally using instruments like the Kibble balance and the Avogadro project's silicon sphere. Consequently, the International Prototype of the Kilogram was officially retired as the primary standard, transitioning from being the definition to a high-mass artifact with a precisely measured uncertainty. It remains a central historical artifact at the International Bureau of Weights and Measures.

Category:Measurement Category:Metrology Category:Standards