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| Replica of International Protoype of Kilogram Source: Wikimedia |
Definition of four base units will be revised at 26th CGPM
Four of the fundamental units are about to change, according
to the announcement made by officials of CGPM or General Conference on Weight
and Measures, at its 26th meet scheduled to held on a stretch of 13th-16th
November, 2018. All these units are proposed to be defined by natural constant
properties or fundamental physical constants such as speed of light and Planck
constant, upgrading previously acknowledged definitions of all four base units. New definition will take into consideration after May, 2019.
Ampere – Unit of Current, Kelvin – Unit of Temperature,
Kilogram – Unit of Mass and Mole – Amount of Substance, are four fundamental
units under radar to get an upgrade.
At present, mass of one kilogram is defined by an object, a
solid cylindrical shaped block composed of 90% Platinum and 10% Iridium
conserved in a bell jar in Paris, France also call IPK or International Prototype of Kilogram. Serving as a calibration tool, IPK has to be compared with its
official copies, in an interval of almost 40 years.
So why there’s need to change the trend? IPK has a
limitation – its weight isn’t constant. Since it has served as an international
standard for more than a century, there’s a slight increase in its weight over
a period of time, thanks to the surface contamination. This serious limitation
affects various scientific assumptions that are based on mass of one kilogram, introducing uncertainty in the results. Interestingly, it is the only unit in
the metric system that is still based on a physical object or an artefact.
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| Kibble Balance, used to measure the Plank Constant in term of IPK Source: Wikimedia |
Other units under scrutiny are ampere, mole and kelvin. Currently ampere is defined as the constant current which produces a certain amount of force, when maintained in two infinite straight conductors of negligible cross-section, placed one meter apart in vacuum. It’s impossible to create wires of infinite length with negligible circular cross-section, making it unachievable to define one ampere practically. Proposed definition of ampere is to define it by taking a fixed numerical value of elementary charge or electrical charge carried by a proton or electron. This will aid scientist to measure one ampere precisely.
Similarly definitions of Kelvin and mole are proposed to be
redefined by changing them into a fixed numerical value of Boltzmann constant
and Avogadro constant respectively. Presently Kelvin is defined as 1/273.16 of
the temperature of triple point of water – point where water exists in its all
three phases. According to current definition of mole, it is no. of elementary
entities as there are atoms present in 0.012 kg of C-12.
Stable basis of measurement and development of measuring
devices with high precision are some reason that has driven this changeover.
This change will go unnoticed or merely understood by most of the people, acknowledge several metrologists involved in bringing this change.
Metric system have been part of International System of Units,
first introduced in France in 1799. Technical limitations resulted in proposal
of artefacts (IPK and IPM etc.) as standards. Several efforts were made in past to make
these fundamental units more precise and eliminate these artefacts. In 1960,
for example, meter was redefined and expressed in terms of wavelength of light
from a specific source which was previously measured in terms of prototype IPM and in 1983, speed of light was updated to 299,792,458
meters per second.
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