Is there like a master ruler? Who decided what a length a cm, mm and inch would be and why?
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Originally it was decided that the metre would be one ten millionth of the distance between the equator and the north pole. Which seems quite reasonable, but it’s realistically rather hard to actually measure accurately.
So instead it was decided to make a reference metre – a metal bar that would be officially decided to be one metre long, so they did.
One they had the official metre, standardising rulers becomes pretty simple – you just compare their length to the reference metre and make sure they match.
In reality it wasn’t quite so simple, as if anyone could walk in of the street, grab the official metre and use it to measure things, it runs the risk of getting damaged or worn down over time and changing length, which would make measuring rather awkward if the length of a metre occasionally shrunk. Instead a select few groups were allowed access to make their own reference metres, those second generation metres were then taken off to various locations and used as a reference to make a third generation metres, which were used to make a larger number of fourth generation metres and so on.
This means that at the highest level, only the biggest and most important government/scientific bodies would ever get access to the highest level reference metres, with more references being made available as they go down the generations and the accuracy theoretically gets a worse tolerance. This means if you want to do high end scientific work, or something like manufacturing high grade measuring equipment it is worth paying the large sums needed to access a low generation reference metre, if you are making mid range tools like a nice quality ruler you could pay less for access to a lower generation reference, and if you are making basic tape measures or school rulers, you can use pretty much whatever as your tolerances will be pretty loose anyway.
So how do we keep rulers standardized? We compare them to the standard reference metre.
Of course it was decided that perhaps basing an entire system of measurement on a metal rod might have its limitations – what happens if someone drops it and damages it? Or how do you use it to accurately measure tiny fractions of a millimetre? And it’s is a bit awkward that if the bar gets warm and the metal expands slightly, the length of a metre changes so it is now temperature specific. So they found a better and more consistent way of starting what a meter is, which is the distance light will travel over a very specific period of time (1/299792458th of a second to be exact).
We do still use the same system of reference metres though, just based on a slightly different standard than picking one specific bar and deciding that it would be exactly a metre.
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