Current in vacuum and gases

A vacuum has almost no charge carriers, so it does not conduct current. When a metal is heated, some of its free electrons obtain enough energy and leave the surface — this is thermionic emission. A vacuum diode is a glass bulb with the air pumped out, containing a heated cathode and an anode. If the anode is given “+” and the cathode “–”, electrons fly from the cathode to the anode and a current appears in the circuit; if the poles are reversed, electrons are pushed back toward the cathode instead of the anode and there is no current. So a diode conducts in one direction only, and was used for rectifying alternating current. An electron beam can be controlled by electric and magnetic fields. Vacuum devices are made of glass and may carry high voltage: opening or breaking them is dangerous, and only specialists repair them.