• The emerging electrons on the other side of the anode then pass through two sets of parallel plates. One set are called the y-plates, and they attract or repel the electrons in the y-axis (vertically). The other set are the x-plates, which affect the x-axis (horizontally). This allows the electrons to be deviated towards any point of the screen.
• When they strike the desired point on the screen, the electrons transfer their kinetic energy into a bright flash (scintillation).
• All of the scintillations construct a picture fast enough to convince the human eye that it is a single image.
All electron movement in the cathode ray tube is in a vacuum. This allows unobstructed paths to avoid collision of particles with gas.
electron gun
vertical deflection plates (y-plates)
horizontal deflection plates (x-plates)
electron beam
fluorescent screen
heated filament
cathode anode
vertical input
FIG. 30.2 The electron path in a CRT Cathode rays:
• travel in straight lines • cause certain substances to emit light • produce X-rays when they strike metal with high velocity • can be deflected in EM fields. (This is shown with the x- and y-plate deflection in the CRT in Fig. 30.2.)
To demonstrate thermionic emission and deflection of cathode rays in EM fields
1. Set up a cathode ray tube. (This may be a basic Crookes’ tube as shown in Fig. 30.4.)
2. As seen in Fig. 30.3, when the tube is turned on, the path of electrons is straight across the fluorescent material. This would be similar to a collimated path of electrons emerging from a cylindrical anode, before deflection.
electron beam
horizontal input
EHT supply
3. Hold a magnet close to the tube and note the deflection of the electron path (Fig. 30.4a). You can also reverse the magnet and note how the path changes with the magnet location and polarity (Fig. 30.4b).
FIG. 30.3
4. You can also place metal plates either side of the beam and apply a voltage to the plates. Again you will see the electron beam move towards one plate and away from the other, showing deflection. Continued
