Units
of Measurement| Units
of Measurement |
In General > s.a. constants [including
Planck units].
* Idea: Now most of them are defined using quantum principles.
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].
@ Related units: in Wheeler & Ford 98, p24 [the barn, term introduced
by Fermi].
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.
Mass
* Kilogram: 1889,
defined as the mass of a piece of Pt-iridium at Sèvres; The only fundamental
unit still defined using a physical artifact, although new definitions have
been advocated.
* Conversion: 1 eV/c2 = 1.8 10–36 kg,
or 1 kg = 5.6
1035 eV/c2.
@ Redefinition of kg: news pw(03)sep;
Robinson pw(04)may;
news pw(05)mar;
news livesci(05)apr;
Robinson SA(06)dec.
Other Quantities
* Electric potential:
The volt is defined using the Josephson
effect.
* Resistance: The ohm
is defined using the quantum Hall
effect.
* Temperature:
The kelvin is currently defined as 1/273.16 of the temperature
difference between absolute zero and the triple point of very pure water held
at a certain pressure; While the triple point technique can define the kelvin
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; The CIPM (International Committee
for Weights and Measures – Bureau 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 planned for 2011.
* Magnetic field: 1 T = 104 G.
References
@ General: Taylor PT(89)aug; Desloge AJP(94)
[em, conversions]; Hebra 03 [history, r pw(04)mar].
@ Systems of fundamental units: NIST reference site;
Jancewicz CEJP(04)
[geometrical]; > s.a. constants [suppression and restoration].
@ And fundamental physics: Bernal et al FP(02) [?].
> Online resources:
Bureau International des Poids et Mesures site;
Internet Encyclopedia of Science pages.
Main page – Abbreviations – Journals – Comments – Other
sites – Acknowledgements
Send feedback and suggestions to bombelli at olemiss.edu – Modified
15 jul 2008