The unit of thermodynamic temperature is defined by two points: one occurring
at the lowest possible temperature (0 K) and one at the triple-point of water
(273.16 K). The thermodynamic temperature scale is defined between 13.8033 K
and 961.78 °C by ITS-90 on the basis of platinum resistance thermometers.
Three other rangers are defined by ITS-90, based on other phenomena. The CIPM
continues to look for better realizations and representations of
thermodynamic temperature.
The original scale devised by Anders Celsius put the 0 mark at the boiling
point of water "on a fair day" and 100 at the temperature of melting, fresh
snow. Thus, the original scale was "upside down" from what we think of it as
being. His original scale evenly divided the length of his hollow glass tube
into 100 parts, so his temperature scale was based on thermal expansion of a
fluid.
Due to the way Celsius temperatures are defined to day,
{t}_C = {T}_K -273.15,
in effect the Celsius temperature scale is defined by the same two points as
the thermodynamic temperature scale or, if one prefers, by one point (the
equation above) and the scaling factor between the kelvin and the degree
Celsius (i.e., 1).
So, the Celsius temperature scale is not defined as most folks think it is, by
the freezing and boiling points of water! Further, it is not given by evenly
dividing a glass tube's length into 100 parts. In the range of normal lab
use, it follows the resistivity properties of platinum.
Office:
Physics Lab Manager, Lecturer
Dept. of Physics and Astronomy
University/College of Charleston
66 George Street
Charleston, SC 29424
843.953.7644 (phone)
843.953.4824 (FAX)