Toroidal cross capacitor for measuring the dielectric constant of gases

We describe toroidal cross capacitors built to accurately measure the diele ctric constant of gases. We tested the capacitors by measuring the dielectr ic 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 calcul ation of the molar polarizability and are limited by the uncertainties of t he capacitance measurements. For argon, the results are consistent with the best previously published measurements of the polarizability and are limit ed by the uncertainties of the pressure measurements. Lessons learned are p rovided. [S0034-6748(00)02407-2].