and derived unit of the International System of units SI
metre is the length of the path travelled by light in vacuum
during a time interval of 1/299 792 458 of second.
is equal to the mass of the international prototype of the
second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine
levels of the ground state of the caesium 133 atom.
ampere is that constant current which, if maintained in two
straight parallel conductors of infinite length, of negligible
circular cross-section, and placed 1 metre apart in vacuum,
would produce between these conductors a force equal to 2
x 10 -7 newton per metre of length.
kelvin, unit of thermodynamic temperature, is the fraction
1/273.16 of the thermodynamic temperature of the triple point
mole is the amount of substance of a system which contains
as many elementary entities as there are atoms in 0.012 kilogram
of carbon 12;
This value corresponds to the number of Avogadro
candela is the luminous intensity, in a given direction, of
a source that emits monochromatic radiation of frequency 540*10
12 hertz and that has a radiant intensity in that
direction of 1/683 watt per steradian.
to the center of a circumference that subtends an arc of equal
length to the beam. 1rad =180°/p
that on a sphere with center in the apex of the angle intercepts
one cap of equal area to that one of a having square equal
side to the beam of the same sphere.
defined independently to the base SI units
atomic mass unit
unit of atomic mass is equal to 1/12 of the mass of an atom
of the nuclide 12C
elettronvolt is the kinetic energy acquired by an electron
in passing through a potential difference of 1V in vacuum
1 u = 1.6605402*10-27
1 eV = 1.60217733*10-19
units of the SI
is a list of the definitions that they have represented the
material of the fundamental quantities of the International
System. From the observation of the preceding definitions
one note the tendency to duty release the units of fundamental
measure from the materials prototype,
for associate it to physical phenomenons. The advantages of
this work are a better and reproducible believer, in as the
Universal Constants are known with least tolerances, and besides
are not subjects to environmental alterations.
in 1799 it was determined from the length of a bar of platinum,
equal to 1/ 40 000 000 of the length of the terrestrial meridian;
in the 1875 is reported to the distance of the two nick mark
on the sample bar of platinum (90%) iridium (10%), preserved
to constant temperature (0°C) in the International Office
of Weights and Measures of Sèvres from Paris; in the 1960 had represented from 1650763.73
lengths of wave, in the vacuum, of the corresponding radiation
to the transition between the levels 2p10
5d5 of the atom of the krypton-86; it in the 1983
is that currently in use.
KILOGRAM: coincident with the mass of 1 dm3
of water distilled to the temperature of 4°C.
before the 1960 it was represented from the 86400a
part of the solar mean day; after the 1960 it was determined
of the fraction of the tropic year to the date of 1900; nel
1967 it is that one currently in use.
before 1982 it was defined from 1/60 of the emitted luminous
intensity from the integral radiator (black body) to the temperature
of solidification of the platinum (2045 K°), in perpendicular
direction to the hole of outlet of the radiation from the
radiator, being the hole area of equal to 1 cm2;
after 1982 it is that one currently in use.
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