TRIPLE POINT CELL
Realization of the kelvin with a triple
point cell at a PTB lab in Berlin. The
cell contains pure water and is used
for calibration of thermometers and
temperature standards which can
be inserted into the central tube.
At exactly 273.16 K (0.01°C), the
three phases ice, water and vapor
coexist in a stable equilibrium.
This delineates one of a number of
defined temperature points our tem-
perature scale relies on.