# New definition of kilogram

Topics Covered:Science and Technology- developments and their applications and effects in everyday life.

### New definition of kilogram

What to study?

For Prelims and Mains: Need for redefining, about the new system, significance and the process of redefining.

Context: The prototype of one kilogram (NPK-57) is now available in India and placed at the National Physical Laboratory, New Delhi.

Background:

Scientists, last year, have changed the way the kilogram is defined. The decision was made at the General Conference on Weights and Measures. The new definitions came into force on 20 May 2019.

How does the new system work?

Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, such as old cars. The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. There is, therefore, a direct relationship between electricity and weight.

So, in principle, scientists can define a kilogram, or any other weight, in terms of the amount of electricity needed to counteract the weight (gravitational force acting on a mass).

What is Planck’s constant?

There is a quantity that relates weight to electrical current, called Planck’s constant – named after the German physicist Max Planck and denoted by the symbol h.

But h is an incredibly small number and to measure it, the research scientist Dr Bryan Kibble built a super-accurate set of scales. The Kibble balance, as it has become known, has an electromagnet that pulls down on one side of the scales and a weight – say, a kilogram – on the other. The electrical current going through the electromagnet is increased until the two sides are perfectly balanced.

By measuring the current running through the electromagnet to incredible precision, the researchers are able to calculate h to an accuracy of 0.000001%. This breakthrough has paved the way for Le Grand K to be deposed by “die kleine h”.

Sources: pib.