
417
Fig. 23.2 Diagrams to illustrate electric focusing
electrons from reaching the phosphor through it, and it is
sufficiently mirror-like to reflect the glow and thereby increase its
brightness as seen from outside the tube.
The only thing that can be done with the tube as described so far is
to vary the brightness of the patch of light on the screen by varying
the grid bias. To make it useful, two more things are needed: a means
of focusing the beam of electrons so that the patch of light can be
concentrated into a small spot; and a means of deflecting the beam
so that the spot can be made to trace out any desired path or pattern.
Both o