Prior to the current definition, the kilogram was defined as being equal to the mass of a physical prototype, a cylinder made of a platinum-iridium alloy, which was an imperfect measure. The kilogram was originally defined as the mass of one liter of water at its freezing point in 1794, but was eventually re-defined, since measuring the mass of a volume of water was imprecise and cumbersome.Ī new definition of the kilogram was introduced in 2019 based on Planck's constant and changes to the definition of the second. This is at least in part due to the inconsistencies and lack of coherence that can arise through use of centimeter-gram-second systems, such as those between the systems of electrostatic and electromagnetic units. SI is a system based on the meter-kilogram-second system of units rather than a centimeter-gram-second system.
Unlike the other SI base units, the kilogram is the only SI base unit with an SI prefix. History/origin: The name kilogram was derived from the French "kilogramme," which in turn came from adding Greek terminology meaning "a thousand," before the Late Latin term "gramma" meaning "a small weight." The changes were intended to improve the definitions of SI base units, not to actually change how the units are used throughout the world. Even though the definition of the kilogram was changed in 2019, the actual size of the unit remained the same. The meter and the second are defined in terms of c, the speed of light, and cesium frequency, Δ νCs. It is currently defined based on the fixed numerical value of the Planck constant, h, which is equal to 6.62607015 × 10 -34 in the units of J Kilogramĭefinition: A kilogram (symbol: kg) is the base unit of mass in the International System of Units (SI). The UK also often uses both pounds and stones when describing body weight, where a stone is comprised of 14 pounds. Many versions of the pound existed in the past in the United Kingdom (UK), and although the UK largely uses the International System of Units, pounds are still used within certain contexts, such as labelling of packaged foods (by law the metric values must also be displayed). This convenience could be the reason that the system was more popular than other systems of the time that used 10, 12, or 15 subdivisions.Ĭurrent use: The pound as a unit of weight is widely used in the United States, often for measuring body weight. This prototype weight could be divided into 16 ounces, a number that had three even divisors (8, 4, 2). It is a system that was based on a physical standardized pound that used a prototype weight. It was updated to its current form in 1959.
The avoirdupois system is a system that was commonly used in the 13 th century. History/origin: The pound descended from the Roman libra, and numerous different definitions of the pound were used throughout history prior to the international avoirdupois pound that is widely used today. The avoirdupois pound is equivalent to 16 avoirdupois ounces. The international avoirdupois pound (the common pound used today) is defined as exactly 0.45359237 kilograms. Definition: A pound (symbol: lb) is a unit of mass used in the imperial and US customary systems of measurement.
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