From the 1979 lecture programme:
Ions are atoms or molecules that have gained an electric charge — by gaining or losing a spare negative electron.
Electric fields can drive ions across, positive ions one way, negative ions the opposite way. An electric field is a region of force from a positive charge to a negative charge, ready to push or pull an ion or any other charged particle. See some patterns of electric fields made with high-voltage machines.
A candle flame contains a swarm of ions, + and -. See the flame driven across by the field between charged plates. See the driven ions forming a tiny measured electric current. Solutions of some salts in water contain ions that can be driven by an electric field. The ions travel across and arrive to provide useful chemical products such as deposits of metal, or bubbles of oxygen and hydrogen.
Electrons are 'chips' torn off the surface of an atom, or boiled off a white-hot wire. Experiments have shown they are all alike, each carrying the same negative charge. A few hundred volts can accelerate them to vast speeds. Then a stream of them responds incredibly quickly and sensitively to deflecting electric fields. That is used in a cathode ray oscilloscope (and in television picture-tubes with magnetic fields to deflect the stream). That will be shown with real electron streams; also with a model of electrified water drops.
A rectifier is a valve which lets electric current through one way but not the opposite way. Watch what it can do with alternating current for a toy train — shown by an oscilloscope as an electronic graph plotter. Returning to ions in gases see how a spark is formed. See the colours of light that atoms can give out as they return from an ionized state or just an excited state. How could such observations possibly tell us of an expanding Universe? We can make a simple atom model of a large positive ion and one or more electrons.