The International Conference of heads of Metrology Institutes held in Sevres, France has decided that the measure of kilogram will no longer be pegged to cylinder. From 2019 onwards, it will be set by value of Planck constant in combination with definitions of meter and second.
Why it was changed?
Till now, the kilogram is the only one among units of measures pegged to real object. It was pegged to a cylinder (shown in fig) made up of 90% platinum and 10% iridium (In India, salt-shaker-sized cylinder weighing exactly a kilogram in National Physical Laboratory was used as ideal measure of kilogram since 1957). It was serving as reference for variety of industries to keep their weights accurate. But this cylinder’s weight in kilogram is known to fluctuate due to surface contamination with time, making it tricky to define its exact mass.
New measure for kilogram
The new measure for kilogram from 2019 will be derived from constants of nature that are all interdependent. It will be set by value of Planck constant in combination with definitions of meter and second. Kibble balance will be used to make accurate measures of Planck constant. Kibble balance is set of scales, which uses force produced by current-carrying wire in magnetic field to balance weight of mass.
Note: In this case, meter and second units themselves are depended on other constants. The second unit is defined by frequency of an atomic transition in cesium-133 and meter depends on second and the speed of light. The Planck constant (denoted h) is a physical constant i.e. quantum of action, central in quantum mechanics.
The new measure for kilogram will provide utmost accuracy in work. It will be no longer be subject to wear and tear and thus and thus, there will be no sources of error in actual reference weight measure.
In last 60 years, several standard units like second, metre, Kelvin, ampere, mole, candela and kilogram ceased to be defined by physical objects. For example, 1 metre earlier was measured by a platinum-iridium bar, but in 1960, it was re-defined as distance travelled by light in vacuum in 1/299,792,458 seconds. From 2018, seven prime units in International System of Units (inner circle) will be defined by seven constants (outer circle).