Don’t worry: new kilogram won’t change your life

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

23 December 2018

Question: How long is one metre? Many may be quick to say respond it is 100 centimetres. But then; how long is one centimetre? If you say it is 10 millimetres, then I will ask: how long is one millimetre? And we can continue until you run out of words!

It is true that 100cm make one metre but that is not the definition of the metre. Centimetres, millimetres, kilometres and the like are just fractions and multiples of a metre. So, the question still remains unanswered: how long is one meter?

Up until 1960, the international standard meter was a metal alloy bar kept it France. National standards officials from around the world would make regular trips to France to standardise their legal units against this bar.

Then in 1960, the scientific community revised the definition of the metre and now used the wavelength of radiation by specified atom (Krypton-86). The advantage now was that national standards officers no longer needed to travel to France to standardise their national units. They could now standardise in laboratories in their own countries.

This definition was changed in 1983 when the scientific community agreed to use the speed of light instead of wavelength. This speed was fixed at exactly 299,792,458 metres per second.

Doing this automatically fixed the definition of one metre: it is the distance that a beam of light travels in 1/299,792,458 of a second. However, that opens up another question: how long a duration is one second?

The answer to that is another long story, but suffice it to say that, in 1967 the scientific community agreed that one second is the duration that elapses while counting 9,192,631,770 vibrations of a Caesium-133 atomic clock. This clock is extremely stable: it varies by less than one second in 100 million years!

Despite these developments in defining the metre and the second, the definition of the unit for measuring mass has remained in the dark ages! Since 1889, the kilogram has been the mass of a metal alloy cylinder kept in a secure vault in France.

Well; now that has changed. Beginning from 2019, the kilogram will be defined in terms of a universal constant – the Planck’s Constant.

This is a constant that tells the amount of energy contained in a quantum of radiation of a given frequency. In November 2018, the International Committee for Weights and Measures agreed on a fixed value of Planck’s Constant, namely, 6.62607015 x 10 raised to the power of -34 kilogram metre-squared per second.

That sounds convoluted but you will notice that the value has three units, two of which are already defined – second and metre. So, agreeing on an exact value for this constant automatically gives us a definition of the kilogram.

But have no fear: this does not change the size of one kilogram: it only changes the process of standardising the unit. Kenya Bureau of Standards officers no longer need to go to France to standardise our kilogram.