# Former definitions of MKS units

The meter and second, at least, have been redefined several times since they became common in science. Sometimes when referencing historical documents, it's useful to know how the units of the time relate to units in the modern day, but I'm finding it hard to locate that information by web searches. Does anyone know a good source (or can provide the information directly)?

For example, when referencing this chart (shared by Livermore National Lab),

I'd want to know what the length of a 1944 meter is in current day meters, and similarly for the second, to check whether they account for the difference in the reported values of the physical constants $c$ and $h$.

• The history of definitions of the meter is clearly explained in Wikipedia en.wikipedia.org/wiki/Metre. – Alexandre Eremenko Dec 7 '15 at 22:48
• @AlexandreEremenko yes, but that doesn't help me as I'm asking for values, not a qualitative history. That information is not in Wikipedia. – David Z Dec 8 '15 at 4:41
• The Wikipedia article gives you values. – Alexandre Eremenko Dec 8 '15 at 13:50
• @AlexandreEremenko where? Can you point me to the part of the article that lists values? – David Z Dec 9 '15 at 4:26
• @VicAche Yes, I looked at that table. That's not what I'm looking for. It gives uncertainties and qualitative definitions, not values. – David Z Dec 9 '15 at 14:26