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Reference Standard
After defining a unit of measurement such
as the kilogram or the metre, scientists
agreed on reference standards that make
it possible to calibrate all measuring
devices. For getting reliable measurements,
all devices such as metre sticks and
analytical balances have been calibrated by
their manufacturers to give correct
readings. However, each of these devices
is standardised or calibrated against some
reference. The mass standard is the
kilogram since 1889. It has been defined
as the mass of platinum-iridium (Pt-Ir)
cylinder that is stored in an airtight jar at
International Bureau of Weights and
Measures in Sevres, France. Pt-Ir was
chosen for this standard because it is
highly resistant to chemical attack and its
mass will not change for an extremely long
time.
Scientists are in search of a new
standard for mass. This is being attempted
through accurate determination of
Avogadro constant. Work on this new
standard focuses on ways to measure
accurately the number of atoms in a welldefined mass of sample. One such method,
which uses X-rays to determine the atomic
density of a crystal of ultrapure silicon, has
an accuracy of about 1 part in 106 but has
not yet been adopted to serve as a
standard. There are other methods but
none of them are presently adequate to
replace the Pt-Ir cylinder. No doubt,
changes are expected within this decade.
The metre was originally defined as the
length between two marks on a Pt-Ir bar
kept at a temperature of 0°C (273.15 K). In
1960 the length of the metre was defined
as 1.65076373 × 106 times the wavelength
of light emitted by a krypton laser.
Although this was a cumbersome number,
it preserved the length of the metre at its
agreed value. The metre was redefined in
1983 by CGPM as the length of path
travelled by light in vacuum during a time
interval of 1/299 792 458 of a second.
Similar to the length and the mass, there
are reference standards for other physical
quantities.
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