# The kelvin scale - how could it have been defined?

## Main Question or Discussion Point

Hey,

I know that the kelvin scale uses the absolute zero as its null point. If so, I wonder why the scale needs to be defined using the triple point of water. I mean, the absolute zero serves the purpose of defining the kelvin scale already, doesn't it?

## Answers and Replies

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xts
You must have two points to define linear scale, not just one.
Celsius used freezing and boiling points of water. Kelvin found one, being fundamental, but still needs the second.

HallsofIvy
Science Advisor
Homework Helper
I was under the impression that the size of a degree Kelvin was defined to be the same as a degree Celcius. If that is true then you would only need a single "base" temperature.

xts
Those two approaches are equivalent, with respect to accuracy.

We may obtain better accuracy using just one point (tripple point of water) than using inherited after Celsius distance between two points (melting and boiling of water at the 1bar pressure), which depend in turn on accuracy of our definition of pressure unit.