New Definitions of Kilogram and Three Other Metric Units to be Used Starting May 2019

The Système International d’unités (SI Unit) or the metric system, as it is better known descended from the decimal system of measurement proposed by John Wilkins, an Englishman in 1668 which was later made the law in France in 1799. It became the official measurement in all countries except Myanmar, Liberia and the United States. Now the International Bureau of Weights and Measures (BIPM) in Paris is set to redefine the units of mass (kilogram), current (ampere), temperature (Kelvin) and the amount of a chemical substance (mole) so that they can, in theory, be reproduced at any time and in any place.

At a meeting in Versailles on November 16th, the world’s measurement bodies are almost certain to approve a resolution that will mean four out of the seven base SI units, including the kilogram and metre, in being redefined in terms of the values of physical constants. This would mean that from May 20th 2019 the constants will themselves be fixed at their current values forever. Any laboratory in the world will then be able to measure, such as, the mass of an object as precisely as the accuracy of their equipment will allow.

Other units of measurements have been redefined like this in the past for this same reason, such as Second (unit of time), Candela (unit if luminosity) and Metre (unit of length). Second redefined in 1967 is now defined by the ticking of a caesium atomic clock, that neither loses nor gains more than a second in 1.4m years rather than pegging it to the rotation of the Earth about its axis, This clock relies on the pulse of microwaves at 9,192,631,770Hz frequency to measure out a second of time, just as the regular oscillations of quartz crystals are used to calibrate electronic watches.

The candela was originally based on the brightness of a candle flame but was redefined in 1979 to be based on the brightness of a source emitting light at a specific frequency in the green part of the spectrum, to which the human eye is most sensitive. In 1983 it was the turn of the metre, which by virtue of the fact that light travels at a fixed speed (299,792,458 metres per second) through a vacuum, was redefined that way.

A computer-generated image of the international prototype kilogram (the inch ruler is for scale). The prototype is manufactured from a platinum-iridium alloy and is 39.17 mm in both diameter and height, its edges have a four-angle (22.5°, 45°, 67.5° and 79°) chamfer to reduce wear. Image credit: Greg L – Originally uploaded to English Wikipedia

The kilogram is the last unit of measure in the international measurement system that’s still defined by a physical object and it will be placed with a more mathematical solution — specifically, scientists want to redefine the kilogram using Planck’s Constant, a value from quantum mechanics in modern physics. The mass of the international prototype kilogram (IPK), the metallic cylinder that is used to define a kilogram is Paris, therefore not universally accessible for comparison. It has also been found to have slightly lost its mass.

When all this is done, the average person won’t notice this change as there will be no difference between the new and old kilogram. This is because the IPK is what is being used to decide the Planck constant.


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