A Transformer Bridge for Accurate Measurement of Relative Permittivities of Gases and Vapors at Low Pressures

A transformer bridge suitable for measurements of the temperature dependence of the relative permittivities of compounds in the gas phase has been developed. The transformer has a 1:1 winding ratio, capacitor arrangement is symmetrical and measurements are made with a substitution technique. The operating frequency is 10 kHz. Capacitance resolution is 2 aF, which is equal to a relative sensitivity of 2 parts in 10 8 . As a test of the performance of the instrument, the relative permittivity of pure nitrogen was measured at 25, 35, and 45°C. The results, reduced to 20°C and 101 325 Pa were identical within any single standard deviation. An estimation of possible systematic errors leads to a final absolute value of ε r for nitrogen at 20°C and 101 325 Pa of ε r = 1 + (547.3 ± 0.3) × 10 -6 .