*Our current picture of the universe is built on the information provided by photons, or light. The quanta of its energy relate to a constant called the Planck’s constant. Now imagine, we have an exact number of photons in a cavity. If we know a balance to measure the mass of these trapped photons, then this idea has a tremendous advantage in defining the kilogram. Yes! This is the basic thought behind the redefinition. As and **when we count photons or atoms precisely in future, it will further improve the measurements of mass. *

The scientific communities are continuously working in the direction of redefining the SI units on the basis of the fundamental constants. This is because of the long-term stability of the The International System of Units (SI). In the twenty sixth meeting of General Conference on Weights and Measures (CGPM) held on Nov 16, 2018, the old definitions of seven physical quantities having their units the second, the metre, the kilogram, the ampere, the kelvin, the mole and the candela stands abrogated and has approved their redefinition. These redefinitions have come into effect on World Metrology Day on 20^{th} May 2019. Now, the kilogram has been defined in terms of the Planck constant. Ever since the redefinition of the metre, the unit of length in 1983, the metrology community proposed of considering redefining the unit of mass in terms of a fundamental constant. Since 1889, the magnitude of the kilogram has been defined and conserved as the mass of an object called the *International Prototype of the Kilogram* "IPK". It is made of 90% platinum and 10% iridium which is a right-circular cylinder having its height equal to the diameter of about 39 millimetres. It has been carefully stored in a triple locked vault at the International Bureau of Weights and Measures in Saint-Cloud near Paris, France. The system of measurement for the entire world depends on this cylinder. According to Richard Steiner, a physicist at the National Institute of Standards and Technology (NIST) "If somebody sneezed on that kilogram standard all the weights in the world would be instantly wrong". For that reason, the official kilogram “IPK” is kept locked inside a secured vault and has been taken out only on three occasions that is in year 1889, 1946 and 1989. Each time, it has been compared to a set of copies. In 1889, the copies and the kilogram weighed the same, but by 1989, the kilogram had drifted apart from the copies. Based on the data, the kilogram appears to weigh slightly less than the copies. Although, the change in weight is very small around fifty micrograms but for the hi-tech industries based on nanotechnology, biotechnology and pharmaceutical industries, where the accurate measurement of mass is very essential, a small variation in mass becomes very crucial.

**Redefinition using the Planck Constant**

The physicists always use precise and unchanging constants to describe the natural world. Initially, the “metre” was equal to the length of a piece of metal kept alongside the kilogram, but in 1983 it was redefined as the distance travelled by light in a vacuum over 1/299,792,458 of a second. As, the speed of light is constant, therefore, this new definition has fixed the “meter”. For this reason, scientists dreamt of redefining the kilogram in terms of a fundamental constant, called the Planck's constant having the symbol *h. *It is a vanishingly small number used in atomic-scale.

In 1900 Max Planck, who was working in Berlin, hypothesized the existence of tiny, indivisible quanta of energy that depends upon a constant known as the Planck constant. This was Max Planck’s most important contribution to the new physics and it laid the foundation for Quantum Mechanics. He realized that his newly discovered fundamental constant, the Planck’s constant along with the other known constants, the Universal Gravitational Constant G and the speed of light c form a unique set of mass, length and time parameters. This unique set is known as the Planck scale and the system of units is known as “Planck’s system”. These three constants are considered as the foundation stone of the physical link between space, time and matter. Moreover, the basis of choosing the Planck constant is that it has become an integral component of modern atomic and subatomic physics and has been universally accepted. Before the redefinition of the kilogram, it is prerequisite to fix the value of Planck constant.

**History and Methods of Measurement of the Planck Constant**

The value of the Planck Constant was evaluated first of all by Millikan in the year 1913 and since 1975, several projects for its evaluation started all over the world. In October 2010, the International Committee for Weights and Measures (CIPM) at Sevres, France submitted a resolution at supreme authority of the International Bureau of Weights and Measures, the General Conference on Weights and Measures (CGPM) that the definition of kilogram will be based on invariant constant of nature that is the Planck constant. Fixing its numerical value to redefine the unit of mass in the SI system was given preference in the twenty fourth meeting of the CGPM on October 2011. The most recent consensus value is obtained using either a Watt balance which is also known as Kibble balance and the X-ray crystal density (XRCD) method. A British physicist and metrologist Bryan Peter Kibble from National Physical Laboratory (NPL), United Kingdom, was the inventor of the Watt Balance in 1975. Kibble balance is a single-pan weighing scale which serves as one of the methods of accomplishing the task of evaluating the value of the Planck constant. The institutes like NPL of UK and National Institute of Standards and Technology (NIST) of the US government pursued the idea of developing the watt balance for defining the unit of mass. Then, in 2009, Watt balance at NPL was sold to National Research Council (NRC) Canada. In 2017, the watt balance was renamed as Kibble balance to honour the inventor who passed away in 2016. In addition to NIST of USA and NRC of Canada, LNE (France), METAS (Switzerland), PTB (Germany), MSL (New-Zealand), NIM (China) and NMIJ (Japan) also joined the project of evaluating the Planck constant using Kibble balance method and XRCD methods. All these Kibble balances are unique in their design and operation and now a day, the idea of building table-top versions of the Kibble balance is getting more popular.

The Kibble balance is an electromechanical apparatus which can works in velocity and weighing mode. It allows the determination of the Planck constant by comparing electrical and mechanical power using two separate experiments. It compares an electromagnetic force produced by a coil in a magnetic field to the weight of a test mass. Then, the coil is moved through the magnetic field of samarium cobalt, which results in the creation of a voltage. The speed of the coil is measured using a laser source. Planck constant is introduced through the use of calibration current and voltage to determine the electric power. These calibration current and voltage are based on two quantum effects. The first is Josephson Effect, which is the phenomenon of flow of current without any applied voltage across a device known as a Josephson junction. The second is the quantum Hall effect which is based upon the experiment performed in 1980 by Von Klitzing for which he was awarded the 1985 Nobel Prize in physics. The weight of a mass can be determined precisely by measuring the speed of coil as well as through the Planck constant which is relating to caliberation of current and voltage.

In parallel to watt balances, another project which allows in determining the value of Planck constant is the Avogadro project initiated by the International Avogadro Coordination. Here, to redefine it in terms of the fixed numerical value of the Planck constant*,* the x-ray-crystal-density (XRCD) method is used. This method has been used for the determination of the Avogadro constant by counting the number of atoms in the Silicon enriched crystal. The experiments have been performed by PTB Germany in coordination with other institutions. By using the above method reversely, the exact value of the kilogram can be determined using sphere prepared from the crystal of silicon.

**Data Compilation of the Planck Constant**

The Committee on Data for Science and Technology (CODATA) was established in 1966. It seeks to improve the compilation, evaluation, storage, and retrieval of important data of science and technology. The CODATA task group on fundamental physical constants provide internationally recommended values of the basic constants of physics and chemistry periodically. The data is based upon recent research on the relevant values of the basic constants of physics and chemistry available at a given point of time. It has considered eight available data: four from Kibble Balance known as NIST-3, NIST-4, NRC-17 and LNE-17 and four from International Avogadro Coordination (IAC) known as IAC-11, IAC-15, IAC-17 and National Metrology Institute of Japan NMIJ-17.

Various experimental measurement of the Planck constant is available from all over the world. Its value have reached a measurement precision of a few parts in 10^{8}.The accuracy of it has now improved to disagreements in the 7th digit with stated uncertainties in the 8th digit. The new definition has eliminated the need for the “IPK”. ** The kilogram(sometimes known as electronic kilogram) has now been defined by taking the fixed numerical value of the Planck constant approved by CODATA 2017 to be 6.62607015 × 10^{-34} Kgm^{2}s^{-1}, where the metre and the second are defined respectively in terms of speed of light and frequency of cesium. Hence, when the Planck constant is fixed and the frequency and the speed of light are already fixed, then the "kilogram is also now fixed”**. Its value has been measured within a relative standard uncertainty of 1.0 × 10

^{-8}.

India is also one of the sixty signatories of the redefinition of the kilogram and has approved change in the definition of the unit of mass and proposes to build a Kibble balance over the next three years that will calibrate weights used in our country. Now, it is theoretically possible to realize the SI unit of mass at any place, at any time and by anyone. Thus, the “IPK” will be functionally retired as definition, but physically it will still be used in the experiments as a standard.