<Tr> <Th> Rankine </Th> <Td> (° R) = (K) × ​ ⁄ </Td> <Td> (K) = (° R) × ​ ⁄ </Td> </Tr> <Tr> <Td_colspan="3"> For temperature intervals rather than specific temperatures, 1 K = 1 ° C = ​ ⁄ ° F = ​ ⁄ ° R Comparisons among various temperature scales </Td> </Tr> <P> The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics . The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI). The kelvin is defined as the fraction ​ ⁄ of the thermodynamic temperature of the triple point of water (exactly 0.01 ° C or 32.018 ° F). In other words, it is defined such that the triple point of water is exactly 273.16 K . </P> <P> The Kelvin scale is named after the Belfast - born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824--1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree . The kelvin is the primary unit of temperature measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude . The definition implies that absolute zero (0 K) is equivalent to − 273.15 ° C (− 459.67 ° F). </P>

Why is kelvin used as the si unit of temperature
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