A Cathode Ray Tube

Author: J.B. Hoag

Fig. 21 J. A cathode-ray tube

The cathode-ray tube in Fig. 21 J consists of a highly evacuated glass tube containing an electron gun at one end and a fluorescent screen at the other end. The fluorescent screen consists of certain chemicals deposited on the inside walls of the end of the tube. The chemicals or " phosphors " are sometimes willemite, calcium tungstate, or phosphorescent zinc sulfide, rendered active by traces of other substances. When these chemicals are struck by fast moving electrons, they emit visible light, the color of which is green, white, yellow, or blue, depending upon the screen material. After the electron impact, the emission of light persists for a more or less short-time interval, usually a small fraction of a second and again depending upon the nature of the screen material. Screens are classified as of long, medium, and short persistence. They are also classified according to the color of the light, the green proving useful for visual observation since the eye is most sensitive to this color, the blue being more satisfactory for photographic purposes, and the white for television applications.

While passing through the tube from the gun to the screen, the electrons pass between the plates of small condensers, called the deflection plates. When one of the plates is made positive and the other negative (by means of an external battery or an applied signal), the electrons are attracted toward the positive plate and are repelled from the negative plate. As seen on the fluorescent screen, the spot of light is moved to a new position away from the central point. When an alternating potential is applied to the deflection plates, the electrons are alternately deflected back and forth to produce a line of light on the screen.

Often there are two sets of deflection plates in a cathode-ray tube as indicated at xx and yy in Fig. 21 J. The two pairs of plates are mounted at right angles to each other and hence can move the electrons back and forth on the screen in the X and Y directions, permitting the production of curves as on an ordinary sheet of graph paper.

The amount of deflection caused by the deflection plates depends upon the speed of the electrons, the voltage applied to the deflecting plates, and their separation. The deflection sensitivity is usually defined as the number of millimeters movement on the screen produced by one volt on the deflection plates. For small tubes, this amounts to about 0.1 mm./volt. It is greater when the anode voltages are smaller, and vice versa.

In certain cathode-ray tubes, the electron beam suffers its deflections before its final acceleration to a high velocity. This offers the advantage that only a small deflecting voltage is required in order to move the electrons large distances across the screen. It is easy to deflect slow electrons by large amounts because they stay for a longer time in the electrostatic field of the deflecting plates.

It is also possible to deflect the electron beam by means of magnetic fields. This is accomplished by passing a current through small coils mounted close to the glass envelope on the outside of the tube, in the same relative positions as the electrostatic deflection plates.

Fig. 21 K. A typical cathode-ray tube circuit. Three-inch 906 tube. R1 and R2 = 1 to 10 megohms. R3 = 0.5 megohms. R4 = 200,000 ohms. R5 = 20,000 ohms. High voltage = 1300 volts d.c.

A typical cathode-ray tube circuit is shown in Fig. 21 K, together with suitable constants for the RCA 906 tube. Changing the position of the sliding contact on the potentiometer R₅ changes the number of electrons passing out of the gun and hence the intensity or brightness of the spot of light on the screen. Changing the sliding contact on R₄ changes the focus or sharpness of the spot on the screen. To some extent, a change of either of these controls affects the other. The sharpest focus is obtained when the beam current is low. The input signals, which are to be observed and studied with this machine, are applied across the resistors R₁R₂, and hence across the deflecting plates. These resistors, which are of one or more megohms, are used to drain off any accumulation of charge on the deflection plates. Often, adjustable d.c. voltages are also applied to the deflection plates so that the spot can be brought to the center of the screen when stray electric and magnetic fields are present. Inasmuch as only a small amount of current and power is required for the high-voltage d.c. supply, a half-wave rectifier may be used with a single 0.5 to 2 µfd. condenser across its output. It will be noted that one side of each deflector, and anode 2, is at ground potential. Anode 1, the cathode, and the grid are all at negative potentials below ground.