Dielectric-constant gas thermometer


As a special version of the gas thermometer, the dielectric constant gas thermometer (DCGT) exploits the fact that the permittivity (dielectric constant) ε of an ideal gas of low number density N/V or low pressure p is given by ε = ε0 + α0N/V (α0 being the static dipole polarisability of a single gas atom). The equation of state thus can also be written in the form

p = kT(ε - ε0)/α0

and, analogous to the gas thermometer,

T/TTPW = (p/pTPW)(εTPW0)/(ε-ε0)

is obtained. The DCGT serves to determine ε-ε0 by measuring the capacitance variation of a suitable gas-filled capacitor when the gas is pumped off.


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