Abstract
A method is described to obtain a voltage proportional to the average electron density in a gas discharge tube of small diameter. The method is based on the well known cavity method. The cavity in which the plasma column is placed is inserted in a feedback circuit known as the Pound stabilization circuit, which was originally devised as a means to stabilize the output frequency of a microwave oscillator on the resonant frequency of a cavity. In the method described here, the oscillator frequency is made to follow the cavity resonant frequency. The voltage which controls the oscillator frequency is made to vary linearly with the frequency and hence with the electron density.