Units of Measurement  

In General > s.a. Measurement [metrology]; Planck Constant [including Planck units].
* Idea: Now most of them are defined using quantum principles.
* SI: There are seven base units of the SI, the second, meter, kilogram, ampere, kelvin, mole and candela; From the Nov 2018 decision at the General Conference on Weights and Measures in Versailles, effective 29 May 2019 all seven will be defined in terms of physical constants.
@ General: Szekeres IJTP(78) [dimensional analysis and fundamental units, axiomatization]; Taylor PT(89)aug; Hebra 03 [history, r pw(04)mar]; Crease pw(09)sep [origin of some unusual units]; Kershaw HSNS(13) [electromagnetic standards of length]; Hehl & Lämmerzahl a1810-proc [general theory].
@ SI: Mills PRS(09), Foster PRS(09) [conventions for writing values of quantities]; Aubrecht TPT(12) [changes coming]; Jayson AJP(14)jan [history, telegraphy and the emergence of the SI]; Newell PT(14); Mohr & Phillips Metr(15)-a1409 [dimensionless (angular and counting) units]; Pavese a1601 [role of fundamental constants]; Mari et al a1604-conf [the New SI, conceptual roadmap for setting up a system of units]; Knotts et al TPT(17) [the New SI]; news pt(18)nov, Phy(19)mar, APS(19)may [revamped system based on constants].
@ Systems of fundamental units: NIST reference site; Jancewicz CEJP(04) [geometrical]; Capitaine & Guinot a0812-proc [astronomical units]; Kitano JMP(13) [mathematical structure]; > s.a. constants [suppression and restoration].
@ And electromagnetism: Desloge AJP(94)jul [conversions]; Carron a1506, Jayson AJP(14)jan-a1512 [historical development of unit systems].
@ And fundamental physics: Bernal et al FP(02) [?]; Buzcyna et al G&C(11)-a1006 [Stoney units and Planck units].
@ Related topics: news lat(10)jul ["hella" as proposed prefix for 1027]; Crease pw(10)jul [Joseph Dombey's misfortunes and the US system].
> Online resources: Bureau International des Poids et Mesures site and possible future changes; Internet Encyclopedia of Science pages; Unit Converter site.

Prefixes for Units (see also Wikipedia page)
symbol prefix value symbol prefix value
d deci- 10–1 da deca- 10
c centi- 10–2 h hecto- 102
m milli- 10–3 k kilo- 103
μ micro- 10–6 M mega- 106
n nano- 10–9 G giga- 109
p pico- 10–12 T tera- 1012
f femto- 10–15 P peta- 1015
a atto- 10–18 E exa- 1018
z zepto- 10–21 Z zetta- 1021
y yacto- 10–24 Y yotta- 1024

Length
* Meter: 1791, defined as one 10 millionth of the distance from the North Pole to the Equator, along the meridian through Paris; 1889?, the length of a platinum-iridium rod at the BIPM (Bureau International des Poids et Mesures) at Sèvres; Now defined using the speed of light as the distance light travels in 1/299,792,458 of a second [@ Bates].
* Conversion: 1 ly = 9.461 × 1015 m (10 trillion km) = 6.324 × 104 au; 1 pc = 3.262 ly = 3.086 × 1016 m; 1 au = 1.496 × 1011 m.
@ General references: Agnoli & D'Agostini phy/04 [origin of meter]; Schücker a0810 [1983 farewell to Sèvres meter].
@ Related units: in Wheeler & Ford 98, p24 [the barn, term introduced by Fermi]; Zois a1203-conf [towards an alternative definition for the meter].

Time
* Second: Originally a definite fraction of the period of the Earth's orbit in 1900; Now defined as 9,192,631,770 times the period for the transition between the two hyperfine levels of the ground state of 133Cs.
@ References: Tal PhSc(11) [measurement accuracy]; Middelmann et al PRL(12) [advances in optical strontium clocks, candidates for a new definition of the second].

Mass
* Kilogram: 1889, defined as the mass of a piece of Pt-iridium at Sèvres, "Le Grand K"; 2012, The only fundamental unit still defined using a physical artifact, although new definitions have been advocated, and an agreement appears to be near; 2018, Agreement on a new definition based on h, effective 20 May 2019.
* Conversion: 1 eV/c2 = 1.8 × 10−36 kg, or 1 kg = 5.6 × 1035 eV/c2.
@ General references: news dm(11)nov [prototype metal cylinder loses 50 micrograms].
@ Redefinition of kg: news pw(03)sep; Robinson pw(04)may; news pw(05)mar; news livesci(05)apr; Robinson SA(06)dec; news pt(10)nov; news wired(11)sep; Becker CP(12) [proposed kg definition based on counting the atoms in a 28Si crystal]; news nat(15)oct [getting closer]; news pw(18)nov [new definition].

Other Quantities > s.a. Mole.
* Electric potential: The volt is defined using the Josephson effect.
* Electric current: The ampere is defined as the current through two parallel conductors 1 m apart that results in a certain force between the two conductors; 2018, The ampere will now be set by the elementary electrical charge e, given by 1.602 176 634 × 10−27 C.
* Resistance: The ohm is defined using the quantum Hall effect.
* Temperature: 0 K = −273.16 °C; The kelvin (K) is currently defined as 1/273.16 of the temperature difference between absolute zero and the triple point of very pure water (with a specific ratio of hydrogen and oxygen isotopes) held at a certain pressure; While the triple-point technique can define the K to better than 1 ppm, the problem with this definition is that it is based on a specific physical situation, which must be reproduced whenever a highly accurate definition of the kelvin is required, and does not work well for very low or very high temperatures, far from water's triple point; The CIPM (International Committee for Weights and Measures – Comité International des Poids et Mesures) wants to define all SI units in terms of each other and the fundamental constants; In the case of the K, this involves the second and Boltzmann's constant, and therefore an accurate value of the kelvin could be obtained from the second, which is known to an extremely high degree of accuracy of about one part in 1016; A redefinition is being planned; 2018, The kelvin will now be set by the set value of the Boltzmann constant.
* Magnetic field: 1 T = 104 G.
@ Redefinition of the ampere: news pw(13)may [based on transport measurements of a single electron in graphene]; Keller & Aumentado Phy(16).


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