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The Redefinition of Scientific Constants


The kilogram, kelvin, and mole, as of the 26th meeting of the General Conference on Weights and Measurements, have new definitions.

The Kilogram

Before the meeting, the kilogram was a set to the Le Grand K, a platinum alloy cylinder that resides in the International Bureau of Weights and Measures in France. It was created as the standard kilogram in 1889, followed by multiple replicas created by each country. In order to preserve the weight of the cylinder, it was placed with great care to protect from any deteriorating forces. One precaution includes having three bell jars surrounding the official cylinder.

The cylinder was taken out once every 40 years to compare the weight of Le Grand K with its official replicas. While checking the weight, metrologists discovered that the difference of weight was around 50 micrograms. Because this slight discrepancy would create slight inaccuracies in measurement, the General Conference on Weights and Measurements voted to redefine the kilogram.

Dr. Stuart Davidson, NPL head of mass metrology, claims that the method of comparing all weights to the standard Le Grand K “…is not acceptable from a scientific point of view. So even though Le Grand K is fit for purpose at the moment, it won’t be in 100 years’ time”.

The Solution

At the GCWM, it has been decided that the kilogram should be defined in terms of electric current. The new system will involve the Kibble Balance using the exerted force of an electromagnet to balance with the 1-kilogram eight. In order to balance, the electromagnet will increase its electric current to find the accurate balance between them. The Planck’s constant is intertwined in this measurement, since the measurement of the constant is possible through the use of Dr. Kibble’s machine. The accurate constant can be used with the electric aspects of the machine for the purpose of the kilogram.

Dr. Bettin proposes another solution involving the unit of moles to describe the kilogram. The creation of a perfect silicon crystal weighing exactly 1 kilogram can be counted in its molecules to determine the silicon molecules per kilogram. Dr. Bettin thinks that “it would be far more simple with the Planck constant.”

Now no longer in the need of a cylinder of comparison, the measurement of the kilogram is now truly constant. When scientists met to standardize the kilogram as the Le Grand K, they tried finding the best measurement system. While searching for an better system of comparison, scientists have used the foundations of the past for a purpose that will continue to serve the world.
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