After years of discussions, tests and experiments, scientists present a new definition of kilogram based on Plank’s constant. The kilogram is a measure that refers to a physical object: a platinum and iridium cylinder kept at the BIPM (Bureau International des Poids et Mesures) of Sèvres in France.

This, despite being kept safe with all possible precautions, undergoes micro-changes, due to dust, heat, cold and other factors, which change its characteristics over time. It is estimated that from 1889 to today its mass has varied about 50 micrograms.

Since the unit of measure must be stable, otherwise you could have complications in many physics and chemistry areas, it was decided to introduce a reference system that was not based on a material object, as is now done for almost all the other units of measurement.

If we take the meter, for example, that was determined by the distance between two parallel segments drawn on a face of a platinum-iridium bar, since 1983 it is defined as the distance that the light travels in a vacuum in a time equal to 3.33564095e^(-9) seconds. The new definition of the kilogram, on the other hand, for those who do not have at least some physics basis, may not be very easy to understand, the “technical” explanation is this: 



Taking a Kibble balance, it balances the weight of the mass sample with the force generated by the current, measured in terms of h through the Josephson effect (to define the potential difference in terms of h / e2) and the quantum Hall effect (to define the impedance in terms of e / h), in a coil placed between the poles of a permanent magnet. Subsequently, the coil is moved with constant velocity and the induced electromotive force is measured, again in terms of the h constant, to determine the proportionality constant between the gravitational force and the electromagnetic force. The mass of the sample is then obtained from its weight by measuring the local gravity acceleration.  


It seems difficult, isn’t it? and in fact it is, but let’s try to explain it in a simpler way:

Taking a Kibble balance , we put on a plate (even if there are no dishes on this balance, but we pretend so) the reference sample and, to counterbalance it, we use a force generated by an electric current flowing in a wire immersed in a magnetic field: thus the kilogram becomes the mass counterbalanced by a certain amount of current, where the Planck constant comes into play.

However as with most of them, Planck’s constant has not yet been defined with absolute precision, even though the National Research Council of Canada has calculated it with an uncertainty of 19 parts per billion, which for now seems more that satisfying but that will soon improve again,  and in any case it remains much more precise than the old cylinder of iridium and platinum, which will soon become just a piece of history, to be exhibited in a museum.

Images sources:



Iridium-platinum cylinder

US national lenght meter

Kibble balance

Study source:

A precise instrument to determine the Planck constant, and the future kilogram

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  1. TRUTH(@i-am)

    Good Stuff !!!
    Did you know that, as you said, the meter is defined by the speed of light. But in fact the speed of light is NOT constant. This started causing a lot of problems with current physics equations that like TSOL to be constant,,,, so they the pegged it (in a variable manner) to the meter, there by eliminating any effects in equations if TSOL has changed.

    Them physics guys,,, love ’em, but some times they cheat because they don’t know what else to do,,,,, take DARK MATTER for example.