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A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce .
system unit unit-code symbol or abbrev. notes sample default conversion combination output units SI: square kilometre: km2 km 2: US spelling: square kilometer: 1.0 km 2 (0.39 sq mi) km2 sqmi; square metre: m2 m 2: US spelling: square meter: 1.0 m 2 (11 sq ft) m2 sqft; square centimetre: cm2 cm 2: US spelling: square centimeter: 1.0 cm 2 (0.16 ...
Measure for how easily current flows through a material siemens (S = Ω −1) L −2 M −1 T 3 I 2: scalar Electrical conductivity: σ: Measure of a material's ability to conduct an electric current S/m L −3 M −1 T 3 I 2: scalar Electric potential: φ: Energy required to move a unit charge through an electric field from a reference point ...
The conversion between different SI units for one and the same physical quantity is always through a power of ten. This is why the SI (and metric systems more generally) are called decimal systems of measurement units. [10] The grouping formed by a prefix symbol attached to a unit symbol (e.g. ' km ', ' cm ') constitutes a new inseparable unit ...
[4] [5] The mouthful was still a unit of liquid measure during Elizabethan times. [6] The principal Egyptian standards from small to large were the ro, hin, hekat , and khar.) [ 7 ] Because of the lack of official definitions, many of these units will not have a consistent value.
Conversions between units in the metric system are defined by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10 −6 metre).
The system can be traced back to the measuring systems of the Hindus [18]: B-9 and the ancient Egyptians, who subdivided the hekat (about 4.8 litres) into parts of 1 ⁄ 2, 1 ⁄ 4, 1 ⁄ 8, 1 ⁄ 16, 1 ⁄ 32, and 1 ⁄ 64 (1 ro, or mouthful, or about 14.5 ml), [19] and the hin similarly down to 1 ⁄ 32 (1 ro) using hieratic notation, [20] as ...
The US customary system has one set of units for fluids and another set for dry goods. The imperial system has only one set defined independently of, and subdivided differently from, its US counterparts. By the end of the 18th century, various systems of volume measurement were in use throughout the British Empire.