Vertical and Horizontal Plates
If two sets of deflection plates are placed at right angles to each other inside a CRT (figure 6-7), the
electron beam can be controlled in any direction. By varying the potential of the vertical-deflection plates,
you can make the spot (beam) on the face of the tube move vertically. The distance the beam moves will
be proportional to the change in potential difference between the plates. Changing the potential difference
between the horizontal-deflection plates will cause the beam to move a given distance from one side to
the other. Directions other than up-down and left-right are achieved by a combination of horizontal and
Figure 6-7.Deflection plate arrangement.
As shown in figure 6-8, position X of the beam is in the center. It can be moved to position Y by
going up 2 units and then right 2 units. Movement of the beam is the result of the simultaneous action of
both sets of deflection plates. The electrostatic field between the vertical plates moves the electrons up an
amount proportional to 2 units on the screen. As the beam passes between the horizontal plates, it moves
to the right an amount proportional to 2 units on the screen.
Figure 6-8.Beam movement on the CRT.