Kilogram

The kilogram (symbol: kg) is the base unit of mass in the International System of Units, abbreviated SI. It is the most widely used unit of mass globally in commerce, industry, and scientific research.

One kilogram is equivalent to one thousand grams (1 kg = 1000 g). Although it serves as a base unit, the kilogram differs from other SI base units in that its name contains a prefix—kilo-, meaning thousand.

History of definition

Originally, in the late 18th century, the kilogram was defined in terms of the mass of water: the mass of one liter of water at a temperature close to its maximum density. This definition was later replaced with one based on a physical object: a cylinder of metal kept under controlled conditions.

For a period, the kilogram was defined as the mass of a specific object called the International Prototype Kilogram, made of an alloy of platinum and iridium and stored at the International Bureau of Weights and Measures in France. Copies were distributed to many countries. The main problem was that this artifact's mass could change slightly over long periods, making the kilogram dependent on a single physical object.

Modern definition

In 2019, a new definition was adopted that does not rely on a physical artifact. Instead, the kilogram is defined using fundamental physical constants, specifically the Planck constant, whose value has been fixed precisely. The definitions of the ampere and the meter are also used. As a result, anyone with appropriate equipment can reproduce the kilogram without needing a reference metal cylinder.

Use and related units

The kilogram forms the basis of many other units: the tonne (1000 kg), the gram, and the milligram. It also appears in the definitions of force (newton) and energy (joule). Some countries, particularly the United States, still use the pound in everyday commerce, but the official standards of science are based on the kilogram.