General Conference On Weights And MeasuresEdit

The General Conference on Weights and Measures (CGPM) is the supreme organ responsible for the governance of the International System of Units. Convened under the framework of the Metre Convention, it brings together delegates from member states and associated states to set policy, approve new definitions, and guide the dissemination of measurement standards worldwide. The CGPM operates in concert with the International Committee for Weights and Measures (CIPM) and the International Bureau of Weights and Measures to ensure that units remain consistent, reproducible, and usable across science, industry, and commerce. The conference’s work directly touches how societies quantify length, mass, time, electric current, temperature, amount of substance, and luminous intensity, among other quantities, with implications for trade, technology, and scientific research.

Since its inception, the CGPM has evolved from a practical project in standardization to a sophisticated system of definitions anchored in fundamental physics. The foundation lies in the Metre Convention of 1875, which established a formal framework for national measurement laboratories to cooperate and for the measurement of the world to be harmonized. Over the decades, the CGPM and its related bodies have progressively moved from artifact-based standards to definitions tied to invariant natural constants, enabling more stable, long-term reproducibility.

History

Origins and the Metre Convention

The CGPM emerged from the need to stabilize measurements across nations engaged in trade, science, and industry. The Metre Convention created a treaty-based mechanism for member states to coordinate measurement practices and to place the most critical standards under a centralized international umbrella. The idea was to prevent regional divergences in units like the metre and the kilogram from disrupting global commerce. The conference and its associated bodies quickly established a network of national laboratories and reference artefacts, including the famous international prototypes for mass and length.

From artefacts to constants

For much of the 19th and 20th centuries, physical artefacts—such as the international prototype metre and the international prototype kilogram—served as the reference standards. As measurement science advanced, concerns about artefact stability and drift prompted a shift toward definitions anchored in immutable natural constants. This culminated in the modern redefinition program, coordinated by the CGPM and implemented through the BIPM and CIPM, which seeks to tie unit definitions to constants that do not depend on a single object or laboratory.

The SI and its evolution

The International System of Units (SI)—the practical realization of the CGPM’s policy—was formalized to bring coherence to measurement practice around the world. The SI defines seven base units, with a framework that permits additional derived units. Over time, the CGPM has ratified phased changes to definitions, often in collaboration with the CIPM and the BIPM. The most transformative recent changes redefined several base units in terms of fundamental constants, a move intended to yield a robust architecture for measurement for generations to come.

Structure and functions

The CGPM is the apex decision-making body for the SI. Its members are national representatives who participate in periodic sessions to discuss and approve proposals that affect the SI’s structure and its applications. While the conference tends toward consensus, formal votes occur when necessary to resolve divergent positions. The CGPM’s decisions are implemented through cooperation with the CIPM and the BIPM, which provide technical expertise, maintain reference standards, and coordinate international comparisons.

National metrology institutes—such as the National Institute of Standards and Technology in the United States, the Physikalisch-Technische Bundesanstalt in Germany, and many others—play a central role in connecting the CGPM’s policy with national measurement systems. These bodies realize definitions, conduct interlaboratory comparisons, and maintain the practical standards used by industry and science. The BIPM in Sèvres, France serves as the central hub, housing the international reference standards and coordinating the global metrology network.

Redefinitions and key units

The International System of Units (SI)

The SI is the modern framework for measurement. It comprises seven base units (metre, kilogram, second, ampere, kelvin, mole, candela) and a set of derived units. The CGPM periodically reviews these definitions to ensure they rest on universally accessible constants of nature and that they remain practical for real-world use. The base units and their interrelations underpin countless applications—from manufacturing tolerances to scientific instrumentation to everyday commerce.

The metre

The metre is defined by fixing the speed of light in vacuum as exactly 299,792,458 meters per second. In practice, this ties the unit of length to an invariant physical constant and the notion of time, via the definition of the second. The speed of light in vacuum is denoted by the symbol c and is a central constant used in a wide range of measurements and technologies. The metre’s modern definition is realized through highly precise optical measurements and interferometry.

The kilogram (and the shift away from artefacts)

The kilogram has moved from being defined by a physical artifact—the international prototype kilogram—to a definition anchored in the Planck constant. By fixing h to an exact value, the kilogram is realized in a way that is independent of a single artefact. This change aimed to eliminate drift that can occur with mass artifacts and to improve long-term stability for precision mass measurements. The story of the kilogram’s redefinition is closely tied to the historic artifact known as Le Grand K, which long served as the international reference for mass but which, over time, exhibited drift relative to other masses.

The ampere, kelvin, and mole

The ampere is now defined by fixing the elementary charge e to an exact value, linking electrical current to the flow of electrons and enabling highly stable electrical measurements.
The kelvin is defined by fixing the Boltzmann constant k to an exact value, anchoring thermodynamic temperature to a fundamental property of matter.
The mole is defined by fixing the Avogadro constant NA to an exact value, grounding the amount of substance in a count of specified elementary entities.

The candela

The candela, the unit of luminous intensity, remains tied to human visual response and is defined by a fixed luminous efficacy for monochromatic radiation at a specific frequency. This definition reflects the interface between physical measurement and perception, balancing objective standards with the way human observers experience light.

Practical implications

Redefinitions are designed to improve robustness, universality, and long-term stability. In practice, national measurement laboratories implement the CGPM’s decisions through sophisticated equipment, calibration protocols, and traceability chains that connect laboratory measurements to the SI definitions via international comparison exercises.

Controversies and debates

Practicality versus universality: Proponents of anchor-based definitions argue that tying units to fundamental constants ensures universality and long-term stability. Critics, including some industry observers, have noted the need for substantial investment in instrumentation and training to realize these definitions in everyday manufacturing and testing. The CGPM has typically sought to balance theoretical rigor with practical feasibility in industry and government laboratories.
Artifacts versus constants: The transition away from artifact-based standards, such as Le Grand K, toward constants, was controversial in some circles that valued the tangible and venerable nature of physical artifacts. Supporters of the change emphasize that constants provide an invariant and accessible foundation, while critics worry about the complexities of realizing exact constants in diverse laboratory environments.
Global equity and access: Because metrology underpins trade and safety, there are ongoing discussions about whether all countries have equal access to the most advanced measurement capabilities and calibration services. The CGPM and the BIPM work to foster international collaboration, training, and interlaboratory comparisons to reduce disparities and ensure consistent measurements across borders.
Cultural and perceptual factors in standards: The definition of certain units, notably the candela, involves human perception. Some observers argue that tying standards to perceptual functions can introduce subjective elements into what is otherwise a pursuit of objective measurement. Advocates of the current approach contend that it reflects the practical intersection of physics and human experience in a rigorous, quantitative framework.
Widening scope and emerging technologies: As measurement needs expand into new domains—nanotechnology, quantum metrology, and advanced materials—the CGPM faces ongoing debate about the pace and scope of changes to the SI. The conference weighs scientific advances against the demand for stable, widely adoptable standards.