Skip to main content
U.S. flag

An official website of the United States government

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.


Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

The Polarizability of Helium and Gas Metrology



James W. Schmidt, R Gavioso, E May, Michael R. Moldover


Using a quasi-spherical, microwave cavity resonator, we measured the refractive index of helium to deduce its molar polarizability Aε in the limit of zero density. We obtained (Aε,meas - Aε,theory)/Aε = (-1.8{plus or minus} 8.4)× 10-6, where the standard uncertainty (8.4 ppm) is factor of 3.6 smaller than that of the best previous measurement. If the theoretical value of Ae is accepted, these data determine a value for the Boltzmann constant that is only (1.8 {plus or minus} 8.4) ppm larger than the accepted value. Our techniques will enable a helium-based pressure standard and measurements of thermodynamic temperatures.
Physical Review Letters


Boltzmann constant, dielectric constant, fundamental constant, helium, index of refraction, microwave resonator, polarizability, pressure standard, temperature standard
Created June 22, 2007, Updated February 19, 2017