Scientists have voted to change the definition of the kilogram, the first change since 1889.
In addition, members of the General Conference on Weights and Measures meeting in Versailles, France also voted to change the definitions of four other International System of Units (SI) measurements - the mole (amount of substance); the kelvin (temperature); the ampere (current) and the candela (luminous intensity).
The new definitions of these four SI units will take effect on May 20, 2019. The new definition of the kilogram will be used by the International Bureau of Weights and Measures (BIPM) and national measurement institutes around the world.
They will be based on the fundamental physical constants of nature, instead of the less precise definitions, these measurements are currently tied to, according to Estefanía de Mirandés of the BIPM located at Saint-Denis outside of Paris, France.
The kilogram will now be defined by the Planck constant. The ampere will be based on the elementary charge (denoted by e or sometimes qe). The Kelvin will be defined by the Boltzmann constant while the mole will be defined by the Avogadro constant. The redefinition of these four base SI units is the most important decision the BIPM has made in the past 100 years.
The kilogram, the most important of these four SI units, was previously defined by a physical cylinder known as the International Prototype of the Kilogram (IPK). The kilogram is equal to the mass of the IPK, which is kept at a sealed vault at the BIPM. The IPK is cylindrical in form. It has a diameter and height of about 39 mm and is made of an alloy consisting of 90 % platinum and 10 % iridium.
To define the kilogram as a physical block is problematic, however. The weight of this block can change over time because tiny amounts as wear and tear cause it to lose mass while dirt causes its mass to increase.
To address this problem, scientists around the world spent nearly two decades discussing how the kilogram should instead be defined in relation to constant measurements of nature. Thus, the Planck constant.
CIPM now defines the kilogram by defining the Planck constant (h), which will be defined as 6.62607015×10-34 joule seconds. The Planck constant can be found by dividing the electromagnetic frequency of a particle of light or photon by the amount of energy it carries.
The Planck constant is usually measured in joule seconds. It can also be expressed as kilogram square meters per second, however.
This approach also effectively defines the kilogram in terms of the second and the meter. We know what a second and a meter is from the other definitions. By adding these measurements to an exact knowledge of Planck's constant, the result is a new and very precise definition of the kilogram.
