In the realm of weights and measures, a noteworthy development took place at the General Conference on Weights and Measures, held in Versailles, France. Representatives from the International Bureau of Weights and Measures voted to redefine the kilogram based on an invariant value known as the Planck constant. The redefinition utilizes a device called the Kibble balance, leveraging this constant value to measure mass through a finely tuned electromagnetic force. Accompanying this adjustment were updated definitions for other units of measurement, namely, the ampere, Kelvin, and mole. These newly agreed definitions by the BIPM will come into effect from May 20, 2019.
Historical Background
Up until the 19th century, the fundamental unit of mass, the kilogram, was based on a physical object known as the International Prototype of the Kilogram. Known as Le Grand K, it is a shining cylinder made of platinum-iridium and housed at the headquarters of the BIPM in Sevres, France. Modern measurements of mass, from micrograms of medicine and gold dust to kilos of fruit or fish, and even tonnes of steel, are derived from this kilogram standard. However, the prototype’s mass isn’t always consistent. Despite being stored in three glass bell jars, it accumulates microparticles of dirt and is prone to atmospheric influences, which can affect its mass.
New Concepts and Measurements
Since 1967, the definition of a ‘second’ turned a new leaf. It was described as the period it takes for a specific amount of energy to radiate from cesium-133 atoms. This interpretation found its way into atomic clocks, forming the basis of all time measurements. Once the second was defined, the definition for ‘meter’ followed suit. The meter is now defined according to a universal constant, namely the speed of light. Today, it is defined as the distance traveled by light in vacuum in 1/299,792,458 of a second. The kilogram then, was redefined using the Planck constant, typically measured in joule seconds but can also be expressed as kilogram square meters per second.
Implication and Benefits
Through accurate measurements of a second and a meter accompanied by a precise understanding of Planck’s constant, a meticulous definition of kilogram is achieved. The new definition of the second fostered seamless communication across the world through technologies such as GPS and the Internet. Experts maintain that changes in kilogram definition will also be advantageous for technology, retail, and health sectors. Despite having minimal immediate practical implications, defining the units based on physical constants allows scientists to measure them at any place or time, at any scale, making way for far more accurate measurements and a solid foundation for science.
Facts about Units and Standards
| Unit | Standard |
|---|---|
| Kilogram | Planck’s Constant |
| Second | Caesium-133 radiation |
| Meter | Speed of light |
| Ampere | Charge per second |
BIPM and CGPM: A Brief Overview
The Metre Convention refers to a treaty that led to the creation of the International Bureau of Weights and Measures, an intergovernmental organization responsible for matters related to units of measurement. Signed in Paris on May 20, 1875 by representatives of seventeen nations, the convention established a permanent organizational structure for member countries to collaborate on matters of measurement. The first CGPM in 1889 authorized the international prototypes for the meter and the kilogram, constituting a comprehensive mechanical unit system. Since then, newer units were added such as the ampere for electric current, Kelvin for temperature, and candela for luminous intensity, culminating in a total of seven base units by 1971. India became a signatory to the Metre Convention in 1957, and is currently one of the 60 Member States of BIPM.
