Demonstration experiment for the initial stage of introducing the displacement current concept

Abstract

A simple demonstration installation is proposed, containing a plate air-dielectric capacitor, an alternating voltage generator, and three identical neon lamps. Two lamps are inserted in the breaks of the wires connecting the capacitor to the generator. The third lamp is located between the capacitor plates. The gas in all three lamps glows, when the AC voltage is applied to the capacitor. This shows that in the experiment, there is a normal conduction current through the lamps. Around this electric current, as it is well known to students, there is a magnetic field. When removing the neon lamp from the capacitor, they observe that the glow of the lamps in the wires does not change. This means that a small neon lamp practically does not affect the field in the capacitor. Therefore, it is natural to think that the time-varying electric field in the capacitor produces a vortex magnetic field, as well as the conduction current flowing through the lamp. So a time-varying electric field can be considered an electric current. Maxwell called it the displacement current to distinguish it from the real conduction current. The displacement current closes an electrical circuit that is broken by a capacitor for direct current. It is advisable to use the demonstration described in the article when introducing the concept of displacement current in the Bachelor’s general physics course.