1. Thermionic Emission – Learning Outcomes
Discuss the principle of thermionic emission.
Discuss the operation of a cathode ray tube.
Demonstrate a cathode ray tube and its deflection of electron beams.
Give uses of cathode ray tubes.

2. Thermionic Emission
Metals have a lot of free electrons – they are not bound to any particular atom, but cannot easily leave the metal because it is positively charged.
When the metal is heated (to ~800 oC), some electrons have enough energy to escape the metal.
Thermionic emission is the emission of electrons from the surface of a hot metal.

3. Cathode Ray Tube
Cathode ray tubes make use of thermionic emission to create “cathode rays” – streams of electrons.
A cathode is heated, causing thermionic emission.
The electrons are accelerated by the anode.
The anode has a gap in it, so some electrons pass through without colliding.
Electric plates or electromagnets are used to direct the beam of electrons.

4. Cathode Ray Tube

5. To Demonstrate a Cathode Ray Tube
Connect a cathode ray tube to a high voltage power supply.
Connect electric plates across the path of the cathode ray.
Note a deflection towards the positively charged plate.
Replace the electric plates with magnets so that a magnetic field flows across the cathode rays.
Note a deflection perpendicular to the magnetic field.

6. Uses of a Cathode Ray Tube
CRTs used to be the most popular tv technology. They need to be quite long to accelerate and direct the rays, and have largely been replaced by LCDs, plasmas and LEDs.
CRTs are often used as the display in oscilloscopes.
In particular, CRT oscilloscopes are used in electrocardiograms (ECGs, display heart rhythms) and electroencephalogram (EEG, display electrical signals in the brain).