The Dielectric Constant of Air at Radio Frequencies

A radio frequency method has been used to measure the dielectric constant $K$ of dry air free from C${\mathrm{O}}_2$, the value obtained being 1.0005893 for standard conditions of temperature and pressure. The probable error in $K-1\mathrm{}$ due to accidental variations is 0.34 percent. The method is a modification of the usual heterodyne beat arrangment. Capacity changes produced when the pressure in the test condenser is changed are compensated by a suitable condenser in parallel. The beat note frequency is compared with that of a fork by means of Lissajous' figures. The test condenser is made of invar to avoid temperature effects. Short connecting wires are used to minimize lead-inductance effects. A few preliminary measurements made with a large direct current voltage superimposed on the high frequency voltage across the condenser indicate no change in the dielectric constant of air, hydrogen, or ammonia. There is some indication that a discharge through the gas decreases its dielectric constant but the effect is probably spurious.