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| Centimetre–gram–second | |
|---|---|
| Name | Centimetre–gram–second system |
| Quantity | Systems of units |
| Units | centimetre, gram, second |
| Introduced | mid-19th century |
| Official | historical |
Centimetre–gram–second is a family of coherent metric systems of units that used the centimetre, gram and second as base units and served as an international practical framework for physics, engineering and astronomy during the 19th and early 20th centuries. It influenced the development of electromagnetic theory, thermodynamics and fluid dynamics through work by figures associated with École Polytechnique, Royal Society, Cavendish Laboratory and national standards laboratories including Physikalisch-Technische Bundesanstalt and National Institute of Standards and Technology. Prominent contributors who formulated or used CGS concepts include James Clerk Maxwell, Hendrik Lorentz, Heinrich Hertz, Oliver Heaviside and Lord Kelvin.
The CGS concept emerged from early metrication debates following the adoption of the metre and gram in Revolutionary France and the subsequent international discussions at institutions such as the Bureau International des Poids et Mesures and scientific gatherings in Paris, London, and Berlin. Influential publications by Gauss and Bessel promoted measurement of terrestrial magnetism and gravity using centimetres and grams, while Maxwell introduced field equations that encouraged coherent units; contemporaries like Stokes, Kirchhoff, Rankine and G. G. Stokes applied CGS in hydrodynamics and heat. Standardization efforts by bodies including International Committee for Weights and Measures and national metrology institutes led to formalizations and alternative electromagnetic formulations proposed by Giorgi and debated at conferences attended by delegates from United Kingdom, United States, Germany, France and Russia.
CGS defined three mechanical base units: the centimetre for length, the gram for mass, and the second for time; these underpinned derived units such as the dyne for force, the erg for energy, and the poise for dynamic viscosity. Notable derived units include the barye for pressure, the stokes for kinematic viscosity, and the gal for acceleration used in geodesy and astronomy by observers at Greenwich Observatory and institutions like the Royal Observatory, Greenwich. Practitioners such as Jean-Baptiste Biot, Pierre-Simon Laplace, Joseph-Louis Lagrange and Adrien-Marie Legendre used CGS-derived measures in analytical mechanics and celestial mechanics, while Osborne Reynolds and Ludwig Prandtl applied CGS units in fluid dynamics and boundary-layer theory.
Electromagnetic theory in CGS spawned several incompatible variants: the electrostatic units (ESU), the electromagnetic units (EMU), and the hybrid Gaussian system favored in theoretical physics. ESU, developed in laboratories like Kew Observatory and used by Coulomb-style torsion experiments, measured charge in statcoulombs and yielded units such as statvolt and statampere; EMU followed conventions from Ampère's work leading to abampere and abvolt. The Gaussian system, promoted in papers by H. A. Lorentz and adopted in texts by Lev Landau and Evgeny Lifshitz, combined ESU and EMU definitions and simplified Maxwell's equations at the cost of differing dimensions for electrical quantities relative to mechanical ones. Debates involving Oliver Heaviside, Hermann von Helmholtz, Friedrich Kohlrausch and Alessandro Volta shaped which variant researchers used in electrodynamics and early radio experiments by Guglielmo Marconi and Heinrich Hertz.
Dimensional analysis in CGS expresses mechanical quantities with base dimensions L, M, T; conversion to the International System of Units (SI) uses exact factors: 1 cm = 0.01 m, 1 g = 10^−3 kg, 1 s = 1 s, so 1 dyne = 10^−5 newton and 1 erg = 10^−7 joule. Electromagnetic conversions require rationalization and a choice of permittivity and permeability conventions; mapping statcoulomb or abampere to coulomb and ampere invokes factors involving the speed of light c and 10^n multiples discussed by Max Planck and committees of the International Electrotechnical Commission. Conversion procedures appear in metrology reports from BIPM and catalogues from National Physical Laboratory and depend on whether one translates ESU, EMU, or Gaussian quantities to SI's ampere-based definitions.
CGS units dominated laboratory physics, classical electrodynamics, astrophysical literature and certain engineering fields well into the 20th century, appearing in journals associated with Philosophical Transactions of the Royal Society, Annalen der Physik, Physical Review and publications from Royal Society of London and Proceedings of the Royal Society A. Astronomers at Mount Wilson Observatory, Palomar Observatory, and Harvard College Observatory used CGS units in reporting luminosities and surface brightness; geophysicists at U.S. Geological Survey and Geological Survey of India used the gal. Theoretical physics pedagogy by Albert Einstein, Paul Dirac, Wolfgang Pauli and Erwin Schrödinger often employed Gaussian CGS for simplicity in field equations, influencing generations of students in institutions such as University of Cambridge, Princeton University, ETH Zurich and University of Göttingen.
Criticism focused on the multiplicity of electromagnetic subsystems, inconvenient decimal prefixes for engineering scales, and confusion when mixing mechanical and electrical dimensions; reviewers from IEEE, International Union of Pure and Applied Physics and national standards committees argued for a single coherent international system. The Giorgi proposal and subsequent adoption of the ampere as a base unit culminated in the practical universal adoption of SI by resolutions of the General Conference on Weights and Measures and the consolidation of standards at BIPM and ISO. By mid-to-late 20th century, textbooks, journals and standards in United States, United Kingdom, Japan and Soviet Union transitioned to SI, although CGS survives historically in archival literature and specialized subfields documented in the holdings of Royal Society Library and institutional archives.
Category:Units of measurement Category:Metrology Category:History of science