Friday, 19 July 2019

A Kibble Balance at the National Physical Laboratory by Guest Blogger John Faulkner

A Kibble Balance at the National Physical Laboratory, Teddington, Surrey

In 1972, a scientist working at the National Physical Laboratory in Teddington Surrey invented an instrument now known as the Kibble Balance. The purpose of Dr Kibbles balance was to resolve a growing problem facing metrology (the science of measurement). 

The modern world was in need of an agreed standard to measure mass to the highest possible precision and be universally available.

Until 16th November 2018, the world standard mass was a kilogram weight stored at the International Bureau for Weights and Measures in Paris. Copies of this mass in different countries were deteriorating and diverging in value over time. The standard was inconsistent. 

The Kibble balance uses an electromagnet to hold a weight in equilibrium. The electromagnet current is adjusted to counterbalance the force of gravity that attracts the mass of the weight down. Once balanced two high precision measurements are taken from the electromagnet, the electric current and its magnetic field. The results are then used to calculate the value of the mass. The equation makes use of some naturally occurring physical constants to produce a result. In the case of the balance, one of the constants is - Planks Constant, that is used in atomic theory known as Quantum Physics. 

Following measurement, if the calculation results in Planks Constant then the measured Mass is 1 kilogram. On World Metrology Day 16th November 2018 an internationally agreed value for Planks Constant was confirmed and so the Kibble Balance could be used to define a kilogram. 

The original mass in Paris and it's copies are now historical objects. 

Metrology uses The International System of Units (SI). 

There are seven base units to measure things: length-metre (m), mass-kilogram (kg), time-second (s), temperature-kelvin (K), electric current - ampere (A), amount of a substance - mole (mol) and luminous intensity of light-candela (cd).

- John Faulkner 

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