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Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. According to Schadow and McDonald, [ 1 ] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers.
For example, 5 km is treated as 5000 m, which allows all quantities based on the same unit to be factored together even if they have different prefixes. A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, 1 km 2 denotes 1 km × 1 km = 10 6 m 2, not 10 3 m 2.
The micrometre (SI symbol: μm) is a unit of length in the metric system equal to 10 −6 metres ( 1 / 1 000 000 m = 0. 000 001 m). To help compare different orders of magnitude , this section lists some items with lengths between 10 −6 and 10 −5 m (between 1 and 10 micrometers , or μm).
A derived unit is used for expressing any other quantity, and is a product of powers of base units. For example, in the modern metric system, length has the unit metre and time has the unit second, and speed has the derived unit metre per second. [5]: 15 Density, or mass per unit volume, has the unit kilogram per cubic metre. [5]: 434
The International System of Units, or SI, [1]: 123 is a decimal and metric system of units established in 1960 and periodically updated since then. The SI has an official status in most countries, including the United States , Canada , and the United Kingdom , although these three countries are among the handful of nations that, to various ...
Multiples and submultiples of metric units are related by powers of ten and their names are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units. In the early metric system there were two base units, the metre for length and the gram for mass. The ...
"The metric system is for all people for all time." (Condorcet 1791) Four objects used in making measurements in everyday situations that have metric calibrations are shown: a tape measure calibrated in centimetres, a thermometer calibrated in degrees Celsius, a kilogram mass, and an electrical multimeter which measures volts, amps and ohms.
The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959. [2] [10] Common imperial units and U.S. customary units of length include: [11] thou or mil (1 ⁄ 1000 of an inch) inch (25.4 mm) foot (12 inches, 0.3048 m) yard (3 feet, 0.9144 m)