![]() ![]() The unit of time should be the second the unit of length should be either the metre or a decimal multiple of it and the unit of mass should be the gram or a decimal multiple of it. For any given quantity whose unit has a special name and symbol, an extended set of smaller and larger units is defined that are related by factors of powers of ten. These derived units are coherent, which means that they involve only products of powers of the base units, without empirical factors. ![]() For quantities derived from the fundamental base units of the system, units derived from the base units are used-e.g., the square metre is the derived unit for area, a quantity derived from length. The definition of base units has increasingly been realised from natural principles, rather than by copies of physical artefacts. Each of the fundamental dimensions of nature is expressed by a single base unit of measure. The historical evolution of metric systems has resulted in the recognition of several principles. ![]() Adopting the metric system is known as metrication. The historical development of these systems culminated in the definition of the International System of Units (SI) in the mid-20th century, under the oversight of an international standards body. The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. A kilogram mass and three metric measuring devices: a tape measure in centimetres, a thermometer in degrees Celsius, and a multimeter that measures potential in volts, current in amperes and resistance in ohms. ![]()
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