if you were checking if a ruler was correct what would you measure it with and how would you know that one was correct

Question: if you were checking if a ruler was correct what would you measure it with and how would you know that one was correct
- Keywords:
  measure,
  ruler
Asked by anon-208661 to Tori, Titus, Stuart, Hannah, Gill, Alessandro on 7 Mar 2019.
- Gill Harrison answered on 7 Mar 2019:
  
  Having spent too many years as a radiographer, I know that my index and middle finger = 2.5cm (one inch in old radiography terms). I still often use that to work out a rough estimate of length when there’s no ruler around.
  
  Probably doesn’t really give you a sensible answer, but that’s because I don’t actually know!
- Hannah Dalgleish answered on 7 Mar 2019:
  
  You could check it against fifty other rulers (or more, if you’re especially keen) and if they all agree then I would trust it.
- Stuart Higgins answered on 8 Mar 2019:
  
  I would check it first with lots of other rulers to see if they all said the same thing (@Hannah: I genuinely didn’t read your answer first, so I’m very happy we said the same thing!). Hmm how else could we check? I have a friend who works at the National Physics Laboratory, which is responsible for saying how long rulers should be. So I would go to visit my friend and they have a machine which measures how fast light travels to decide. The speed of light (when there isn’t any air in the way) is constant everywhere in the universe, and physicists define the meter as the distance light travels in 0.00000000333564 seconds.
  National Physics Laboratory website: http://www.npl.co.uk/si-units/metre/

Question: if you were checking if a ruler was correct what would you measure it with and how would you know that one was correct

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