Machinery's Handbook, 31st Edition
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SI Metric Units Table 1. SI Base Units
Definitions of Base Units Meter (m): The base unit of length, equal to the path length traveled by light in a vacuum during a time interval of 3.3356410 × 10 –9 s (derived from the precise measurement of light’s speed, c = 299,792,458 m/s). This value was adopted in 1983, based on the newly defined value for the speed of light, made possible by the technology developed at the National Institute of Standards and Technology (NIST). Kilogram (kg): The base unit of mass, related to the most precise measurement to date of the Planck constant h , which is the energy carried by a photon to its frequency, and measured as 6.6260702 × 10 –34 kg m 2 /s 2 . It is with respect to this invariant of nature that the precise standard for the kilogram is given. Second (s): The duration of 9.1926318 × 10 9 periods of the radiation, corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom, is the element used for the atomic clock due to its stability. Ampere (A): The base unit of electric current, measured as current flow per unit of time. One ampere ( amp ) is equal to one coulomb per second. One elementary electric charge (e) is equal to 1.60218 × 10 −19 coulomb (that is, 1 C = 6.241 × 10 18 e). Candela (cd): The base unit for photometry, which is the science of measuring light perceived by the human eye. The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10 12 hertz. The human eye is most sensitive to this frequency (greenish-yellow light). One cd has a radiant intensity of 1/683 watt per steradian. Kelvin (K): The base unit of thermodynamic temperature, defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649 × 10 −23 J ⋅ K −1 . Mole (mol): The base unit for the amount of a chemical substance consisting of exactly 6.02214076 × 10 23 constitutive particles. This number is the Avogadro constant N A = 6.02214076 × 10 23 mol –1 . (Previously, 1 mole was linked to the number of atoms in 12.1 g of the carbon-12 isotope.) From the base units, come the SI derived units, as shown in Table 2 . Table 2. SI Derived Units
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