Metric system

Metric system. The metric system is an internationally adopted decimal system of measurement. It is the modern form of the International System of Units, the definitive standard used in global science, commerce, and industry. The system is defined by seven base units and a standardized set of decimal prefixes, facilitating coherent calculations and international standardization.

History

The system originated in France during the French Revolution, with its development championed by figures like the Marquis de Condorcet and Pierre-Simon Laplace. The French Academy of Sciences was tasked with creating a "rational" system, leading to the definition of the metre and the kilogram in the 1790s. Adoption was formalized by the French First Republic with the Metric Act of 1795, and its use was later promoted across Europe by Napoleon Bonaparte. The Treaty of the Metre, signed in Paris in 1875 by nations including the United States and German Empire, established the International Bureau of Weights and Measures in Sèvres to oversee the system. This body convened the General Conference on Weights and Measures, which has refined the definitions, culminating in the modern International System of Units based on fundamental constants.

Units and prefixes

The system is built upon seven base units: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. All other units, like the newton for force or the pascal for pressure, are derived coherently from these bases. A universal set of decimal prefixes, standardized by the International Bureau of Weights and Measures, scales these units. Common prefixes include kilo- (10³), centi- (10⁻²), milli- (10⁻³), and mega- (10⁶), allowing expression of quantities from the subatomic to the astronomical without conversion factors.

Usage around the world

The system is the official system of measurement for nearly every country, with notable holdouts including the United States, Liberia, and Myanmar. It is the mandated language of measurement in international science, used by organizations like CERN and NASA, and in fields from Olympic Games athletics to International Civil Aviation Organization regulations. Major economies like China, India, Japan, and the European Union conduct all official business using it. Even in countries like the United Kingdom and Canada, which use Imperial units in some public contexts, the system is the standard for science, medicine, and most government data.

Standards and conventions

Global standards are maintained by the International Bureau of Weights and Measures under the authority of the Metre Convention. The definitions of the base units, as codified in the International System of Units, are now based on invariant constants of nature, such as the speed of light and the Planck constant. Conventions for its writing are established by the International Organization for Standardization, including the use of a space as a thousands separator and a comma or point as a decimal marker. The International Vocabulary of Metrology, published jointly with the International Electrotechnical Commission, ensures unambiguous terminology.

Relation to other systems

The metric system superseded a plethora of local and national systems, such as the English units in Britain and various Obsolete German units in the German Confederation. It remains distinct from, though related to, the International System of Quantities framework. While the United States customary units are legally defined in terms of it, everyday use there remains non-metric. The system has also influenced specialized systems, including the Gaussian units used in electromagnetism and the geometrized unit system employed in general relativity. Its principles of decimalization were a direct inspiration for other decimal systems, such as the French franc currency.