Kilogram (WATT Balance)

International Bureau of Weights and Measures (BIPM) had decided that the kilogram will no longer be pegged to this cylinder made of 90% platinum and 10% iridium.


In the last 60 years, several standard units — the second, metre, ampere, Kelvin, mole, candela and, the kilogram — have all ceased to be defined by physical objects. One metre, for instance, was a platinum-iridium bar of that measure. In 1960, the metre was defined as the distance travelled by light in vacuum in 1/299,792,458 seconds.

  • Reason: In essence, the units were freed from being defined on the basis of artefacts, as these being objects, were subject to wear and tear and sources of eventual error.
  • The new artefacts, ought to derive from the constants of nature that are all interdependent. These include constants such as the Planck constant — the ratio of the electromagnetic radiation from a photon to its frequency — and the charge of an electron.

International prototype kilogram is a cylinder of platinum and platinum-iridium alloy stored at the International Bureau of Weights and Measures (BIPM). It was formed in 1875 at the Convention of the Meter as the embodiment of the official weight of a kilogram. Within 10-15 years, about 40 copies were produced and distributed to the nation signatories of the Meter Convention. Although these are stored in filtered-air environments at a constant temperature and pressure, they have picked up tens of milligrams of contaminants each decade. Thus the kilogram objects around the world no longer have exactly the same weight. These errors become magnified at both ends of the scale, leading to critical inaccuracies in descriptions of tiny objects and massive errors when measuring things at cosmic scales. That’s why in October 2017, in Paris, BIPM reviewed plans to overhaul the manner in which the standard values for the kilogram is calculated. Till recently, the kilogram was the only one among the units still pegged to a real object. After a formal vote in 2018, the kilogram will be redefined in terms of the Planck constant, the second and the metre.

Kibble balance (or watt balance)
  • The new definition of kilogram will be calculated by using the Kibble balance named after the British physicist Bryan Kibble.
  • It is an electromechanical weight measuring instrument that measures the weight of a test object very precisely by the strength of an electric current and a voltage.
  • It is a set of scales, which uses the force produced by a current-carrying wire in a magnetic field to balance the weight of a mass.
  • Through this, accurate measures of the Planck constant — the fulcrum of several of the standard units — can be made.
Watt Balance

Watt balance compares mechanical power with electromagnetic power using two separate experiments. First, a current is run through a coil inamagnetic field to create a force that counterbalances a known physical mass. Then, the coil is moved through the field to create a voltage. By measuring the speed as well as experimental values that relate the voltage and current to Planck’s constant, scientists can precisely determine the weight of mass in kilograms.