Something I’ve always been curious about – how do we keep units of measurement like centimeters, kilograms, seconds, etc the exact same over time? If we use measurement tools to create other measuring tools over the span of centuries, how do we prevent slight deviations from the original measurements in the long run? For example, when we measure a kilogram today, is that still the precise mass of the original kilogram?
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This was done by making standard objects which we could use to define the unit. So a meter would be defined using a meter stick made of a resistant alloy and kept in consistent conditions. Whenever the meter stick would change so too would the definition of the meter. Occationally the stick would be used to make or confirm replicas and scale models that were used to calibrate measuring instruments. The most famous of these objects is the “big K” which was until quite recently the definition of a kilogram. However current definitions of the units is based on physical properties of the universe like the speed of light in vaccuum and avrogados constant. These are thought to never change and so defining these as fixed values will allow us to recreate the original measurements even if we have lost the defining standard object or if that object have changed in some way.
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