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It derives from the Old French grosse douzaine, meaning "large dozen”. [5] The continued use of these terms in measurement and counting represents the duodecimal number system. [6] This has led groups such as the Dozenal Society of America to advocate for wider use of "gross" and related terms instead of the decimal system. [7] [8]
The base units are defined in terms of the defining constants. For example, the kilogram is defined by taking the Planck constant h to be 6.626 070 15 × 10 −34 J⋅s, giving the expression in terms of the defining constants [1]: 131 1 kg = (299 792 458) 2 / (6.626 070 15 × 10 −34)(9 192 631 770) h Δν Cs / c 2 .
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.
In this example, the last digit (8, contributing 0.8 mm) is likewise considered significant despite its uncertainty. [1] Therefore, this measurement contains four significant figures. Another example involves a volume measurement of 2.98 L with an uncertainty of ± 0.05 L. The actual volume falls between 2.93 L and 3.03 L.
The definition of base units has increasingly been realised in terms of fundamental natural phenomena, in preference to copies of physical artefacts. A unit derived from the base units is used for expressing quantities of dimensions that can be derived from the base dimensions of the system—e.g., the square metre is the derived unit for area ...
120 μm – the geometric mean of the Planck length and the diameter of the observable universe: √ 8.8 × 10 26 m × 1.6 × 10 −35 m 120 μm – diameter of a human ovum 170 μm – length of the largest mammalian sperm cell (rat) [ 102 ]
A unit of measurement, or unit of measure, is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. [1] Any other quantity of that kind can be expressed as a multiple of the unit of measurement. [2] For example, a length is a physical quantity.
The measurement of time is unique in SI in that while the second is the base unit, and measurements of time smaller than a second use prefixed units smaller than a second (e.g. microsecond, nanosecond, etc.), measurements larger than a second instead use traditional divisions, including the sexagesimal-based minute and hour as well as the less ...