Kilo-

Kilogram

The kilogram (symbol: kg) is the base unit of mass in the International System of Units (SI), the modern form of the metric system. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10⁻³⁴ kg⋅m²⋅s⁻¹, given the definitions of the meter and the second. Prior to this definition, which was adopted in 2019, the kilogram was defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium alloy cylinder stored in an environmentally monitored safe at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

History

The kilogram was originally defined in 1795 as the mass of one liter of water at the freezing point. This was a practical choice that made the gram the base unit of mass in the metric system. However, the difficulty in precisely measuring the volume of water led to the adoption of the IPK as the standard of mass in 1889. The IPK served as the standard for over a century until the redefinition in 2019, which aimed to tie the kilogram to fundamental physical constants.

Redefinition

The redefinition of the kilogram in 2019 was a significant milestone in the history of measurement. It marked the transition from a physical object as the standard of mass to a definition based on fundamental constants of nature, specifically the Planck constant. This change was made possible by advances in technology, such as the Kibble balance, which allows for the precise measurement of the Planck constant.

Implications of the Redefinition

The redefinition of the kilogram has several important implications. It ensures greater stability and universality of the unit of mass, as the Planck constant is invariant across time and space. This change also facilitates more precise measurements of mass at both very large and very small scales, which is crucial for scientific research and technological development.

Units Derived from the Kilogram

The kilogram is the base unit of mass in the SI system, and several other units of mass are derived from it. These include the gram (1/1000 of a kilogram), the tonne (1000 kilograms), and various other units used in specific contexts, such as the milligram and microgram.

See Also

• International System of Units
• Meter
• Second
• Planck constant
• International Prototype of the Kilogram
• Kibble balance