How are do we keep units of measurement the exact same over time?

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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?

In:
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Because we aren’t using tools, in terms of volumes necessarily. One gram is one cubic centimeter of water. We can build from there Bonus fact: One calorie is the amount of energy it takes to raise one cubic centimeter of water by 1 degree.

Recently (and by that I mean over the last decade) the scientific community has moved away from physical standards and have moved to definitions of units of measure based on universal constants. The most recent was the kilogram which was redefined in May 2019 to be based on the Planck constant with other comparable measurements given. [Source](https://www.npl.co.uk/news/science-loses-a-kilogram)

also in a vault there is a weight that is perfectly 1 Kilogram gram and that is also used to calibrate things.
.https://www.npr.org/templates/story/story.php?storyId=112003322

It can also be calculated using avogadro’s number and the molecular make up of an item.

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.

It’s interesting that you use the Kilogram as your example, as that’s recently become a point of contention.

THE Kilogram is a physical entity, kept under lock and key and is the definition of how much mass a kilogram is. BUT since that’s existed for so long it’s actually starting to lose mass on a very small scale. So it’s driving the scientific community to make more choices redefining units of measurement to some known quantity, instead of basing it on some physical standard.

So rather than defining the Kilogram as “the SI unit of mass equivalent to the international standard kept at Sevres near Paris”, we redefined it using the Planck constant. Since the Planck constant is an unchanging value (hence “constant”), we can use it to derive the value of a kg using: h = 6.62607015 × 10^−34 kg ⋅ m^2 ⋅ s^−1

https://puu.sh/Ff2OD/f8ee85d3df.png

By defining 7 primary constants of nature, we can use these constants to define all other units of measurement with unchanging definitions, independent of some man-made prototype or model that could be limited, or unstable.

(the ground state hyperfine structure transition frequency of the caesium-133 atom) ΔνCs = Δν(133Cs)hfs = 9192631770 s−1

(The speed of light) c = 299792458 m⋅s−1

(The Planck Constant) h = 6.62607015×10−34 kg⋅m2⋅s−1

(The elementary charge) e = 1.602176634×10−19 A⋅s

(The Boltzman constant) k = 1.380649×10−23 kg⋅m2⋅K−1⋅s−2

(The Avogadro constant) NA = 6.02214076×1023 mol−1

(The luminious efficacy of monochromatic radiation of frequency 540 X 10^12 Hz) Kcd = 683 cd⋅sr⋅s3⋅kg−1⋅m−2