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TEXT 10

CATHODE-RAY TUBE (Continued)

1. If the electron beam leaving the electron gun were
not deflected, it would produce a luminous spot at the cen­
tre of the fluorescent screen. The intensity of this spot can
be controlled by adjusting the control-grid bias, but it is
also dependent on the fluorescent material coated on the
inside of the tube. Among commonly used fluorescent mate­
rials are zinc silicate, which gives off predominantly green
light; zinc oxide, which gives off a blue color; zinc beryl-
lium silicate or zinc sulphide, which glow yellow; and com-

bination of fluorescent materials, which can be selected to give off nearly white light.

2. Another important consideration in the choice of
fluorescent materials is their afterglow or persistence of'
glow after the electrons have ceased to bombard a spot on
the screen. If the afterglow is less than 0.1 second, the screen
is said to have short persistence; if it is 1 second or more,
it has a long persistence. Between these limits the persistence

"is medium. Short persistence is desirable for rapidly chang­ing images, such as those displayed in television. Long persistence is occasionally of advantage, such as in radar presentations and wave-form comparisons.

3. You can readily see that some means must be provid­ed for removing the electrons from the screen and returning them to the cathode. Otherwise the negative charge on the screen would build up to a point where it would repel arri­ving electrons, and no more could reach it. The method used for removing the electrons is to place a conducting coating of carbon particles, called Aquadag, along the side walls of the tube except the screen and to connect the coating to the cathode or the accelerating anode, providing a ground return to the cathode. Although the coating is not directly connected to the screen, the electrons are removed by means, of secondary emission from the screen and no pile-up occurs. The secondary electrons are collected by the coating and then returned to the cathode.

4. The electron beam may be deflected either electro­statically or magnetically. Let us consider electrostatic de­flection. An electrostatic field can be produced between two metal plates simply by applying a potential difference be­tween them. Electrons entering the field between the def­lecting plates will be bent in the direction of the lines of force, that is, they will be attracted toward the positive plate. The resultant path of the beam will be the net effect between its forward velocity and its deflection.¹ As a result the electron spot is deflected upward or downward depending upon the polarity of the voltage on the plate.

5. Note in Fig. 6 that two pairs of deflecting plates at right angles to each other are set into the path of the elect­ron beam. The vertical deflecting plates move the beam ver­tically up and down, while the horizontal deflecting plates move the beam sideways, to the left or right of the centre.

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