CGS

CGS
Name	CGS
Type	Unit system
Based on	Centimeter–gram–second system of units
Introduced	19th century
Derived from	Metric system
Status	Historical; partially used
Related systems	International System of Units, Imperial units, United States customary units

CGS.

The CGS system is a coherent system of physical units built on the centimeter, the gram, and the second as base quantities. It served as a dominant practical framework for scientific work in fields such as classical mechanics, electromagnetism, thermodynamics, and optics during the late 19th and early 20th centuries. Its adoption influenced standardization efforts involving bodies like the Bureau International des Poids et Mesures, the International Electrotechnical Commission, and national metrology institutes such as the National Institute of Standards and Technology.

Overview

CGS defines base units around the centimeter for length, the gram for mass, and the second for time, producing derived units like the dyne for force and the erg for energy. In electromagnetism, CGS branched into multiple formulations—commonly the Gaussian, electrostatic (ESU), and electromagnetic (EMU) variants—each using different conventions for linking electric and magnetic quantities to mechanical units. The system influenced textbooks by authors such as James Clerk Maxwell, Hendrik Lorentz, and Oliver Heaviside, and was widely employed in research at institutions like the Cavendish Laboratory and the Kaiser Wilhelm Institute.

History

Origins of CGS trace to efforts by metric advocates and metrologists in the 19th century, building on work by Carl Friedrich Gauss and Wilhelm Eduard Weber that tied electrical measurements to mechanical standards. The system gained formal expression in publications and international congresses involving delegates from France, Germany, United Kingdom, and the United States of America. Debates over electromagnetic units involved figures like Heinrich Hertz, Pierre Duhem, and Gustav Kirchhoff and led to multiple CGS electromagnetic conventions. Organizations such as the International Electrotechnical Commission and the Conference Générale des Poids et Mesures played roles in negotiating standards before the gradual shift toward the International System of Units in the mid-20th century.

Units and Definitions

Base units: the centimeter (cm), the gram (g), and the second (s). Derived mechanical units include: - dyne (force) = g·cm·s−2 - erg (energy) = g·cm2·s−2 - poise (dynamic viscosity) = g·cm−1·s−1 Electromagnetic units differ among subsystems: - CGS Gaussian: blends ESU and EMU, employs the statcoulomb and the gauss for magnetic flux density. - CGS-ESU: uses the statampere and defines electric units via electrostatic force. - CGS-EMU: uses the abampere and defines magnetic units via magnetic poles. Constants such as the speed of light c appear explicitly in relations between ESU and EMU quantities, echoing experimental links established by Albert A. Michelson and theoretical formalisms by Lorentz.

Applications and Usage

CGS remained prevalent in theoretical physics and astrophysics in the early 20th century, appearing in papers by Ernest Rutherford, Niels Bohr, and Subrahmanyan Chandrasekhar. Fields that favored CGS include electrodynamics (many classical texts used Gaussian units), plasma physics, and sections of astronomy where convenient numerical scales aligned with astrophysical magnitudes. Laboratories at institutions such as Cambridge University, Princeton University, and the Max Planck Institute historically reported measurements in CGS units, and classic monographs by Paul Dirac and Lev Landau commonly employed CGS conventions.

Advantages and Limitations

Advantages: - Coherence: derived units in CGS are algebraically simple combinations of base units, aiding hand-calculation in mechanics and early electromagnetism. - Numerical convenience: in some subfields, magnitudes of quantities like magnetic fields and cross sections fit compactly into CGS scales (e.g., the gauss versus the tesla). Limitations: - Electromagnetic ambiguity: multiple incompatible CGS subsystems (Gaussian, ESU, EMU) produced confusion across publications and impeded interoperability among laboratories and industries represented by International Electrotechnical Commission standards. - Practicality: industrial and commercial measurement infrastructures in countries using meter-based standards and electrical systems favored larger base units like the kilogram and the ampere, reducing CGS utility in applied metrology. - Conversion complexity: translating between CGS and systems such as the International System of Units requires careful handling of factors involving 4π and c, as highlighted in conversion discussions involving the Committee on Data for Science and Technology.

Transition to SI

The move toward the International System of Units accelerated after mid-20th-century international standardization efforts. Influential meetings of the General Conference on Weights and Measures and recommendations from the International Union of Pure and Applied Physics steered adoption of base units including the meter, kilogram, second, ampere, kelvin, mole, and candela. Transition drivers included industrial demands evident in International Organization for Standardization processes, large-scale electrical engineering projects influenced by the International Electrotechnical Commission, and educational preference for a single coherent global framework. By late 20th century, SI became predominant in journals published by houses such as Elsevier and societies like the American Physical Society.

Notable Variants and Subsystems

Major CGS variants: - CGS-Gaussian: favored in theoretical physics and classical electrodynamics texts by Jackson, J.D.-style expositors and earlier by H. Poincaré papers. - CGS-ESU: electrostatic units, historically used in early electrical experimentation by Coulomb-related researchers. - CGS-EMU: electromagnetic units, connected to magnetism experiments of Gauss and Weber. - Heaviside–Lorentz system: a rationalized variant blending CGS convenience with rationalization akin to later SI rationales, discussed in works by Oliver Heaviside and Hendrik Lorentz. Specialized subsystems appear in disciplines like plasma physics and astrophysics; authors such as E. M. Lifshitz and L. D. Landau present context-specific conventions that readers must map to SI when comparing experimental data.

Category:Systems of units