Toroidal cross capacitor for measuring the dielectric constant of gases

We describe toroidal cross capacitors built to accurately measure the dielectric constant of gases. We tested the capacitors by measuring the dielectric polarizability of helium and argon at 7 and 50 degrees C at pressures up to 3 MPa. For helium, the results are consistent with the ab initio calculation of the molar polarizability and are limited by the uncertainties of the capacitance measurements. For argon, the results are consistent with the best previously published measurements of the polarizability and are limited by the uncertainties of the pressure measurements. Lessons learned are provided. [S0034-6748(00)02407-2].