Definition and realisation of SI base units
Synchronise all the clocks in your computers, car, microwave, iPod, and it’s not very likely they would be telling the same time a year later. Getting out of step shows that the units they are counting with are not all exactly one second long.
So how can we measure a second precisely?
If you’re an athlete trying to beat the world record you really want to know you can trust the measuring instruments involved!
The most obvious way is for there to be things that are definitely, exactly, precisely known: such as something one metre long, lasting one second, having a mass of one kilogram etc.
So somewhere are there really ultimate versions of the kilogram, metre, ampere, Kelvin, mole and second? Yes and no!
There is an international prototype kilogram to which all the kilograms in the world defer. Copies, as similar in mass as possible to the international prototype are kept in various laboratories around the world. The UK’s copy is number 18, and it is kept at NPL.
However, despite being carefully maintained atoms are constantly falling off or sticking to the kilogram. The corresponding changes to mass are tiny but show the kilogram cannot do its job indefinitely.
For this reason metrologists prefer not to use artefacts to define the other six base units. Instead there are definitions which, when turned into reality, can make something which is exactly one metre long or lasts for exactly one second etc.
1 of 4