The electron gun of a cathode ray tube produces
Answer: A
It comprises a heated cathode, a control grid and a set of anodes which accelerate and focus the electrons emitted. Everything downstream of the gun deflects that beam or displays where it lands; the gun itself only makes and shapes it.
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The brightness of the trace on a cathode ray tube is controlled by the potential of the
Answer: B
A more negative grid repels more of the emitted electrons back toward the cathode, so fewer reach the screen and the trace is dimmer. It is exactly the action of the grid in a triode, and the control is marked intensity or brightness on the panel.
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Focusing in a cathode ray tube is achieved by
Answer: C
The shaped fields between the successive anodes act on the diverging electrons much as a glass lens acts on light, bringing them to a point at the screen. Altering the potential of the focusing anode moves that point onto the screen surface.
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The purpose of the aquadag coating on the inside of a cathode ray tube is to
Answer: D
Electrons striking the phosphor knock out secondary electrons, and without a return path the screen would charge negatively and repel the beam. The conducting graphite coating, held at the anode potential, collects them and completes the circuit.
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Persistence of a cathode ray tube screen refers to
Answer: A
A long persistence phosphor holds a trace for a second or more, which suits slow phenomena that would otherwise appear as a moving dot. A short persistence suits fast repetitive signals, where a lingering trace would smear the display.
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The deflection of the beam in a general purpose oscilloscope tube is
Answer: B
Two pairs of plates inside the tube deflect the beam by the electric field between them. Electromagnetic deflection, with coils outside the neck, is used in television and radar tubes, where large deflection angles matter more than high frequency response.
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The deflection sensitivity of a cathode ray tube is expressed in
Answer: C
Sensitivity is the deflection produced per volt applied to the plates, so its unit is a length divided by a voltage. Its reciprocal, volts per millimetre, is called the deflection factor, and confusing the two is a standard examination trap.
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The deflection sensitivity of a cathode ray tube can be increased by
Answer: D
Sensitivity is proportional to the plate length and the tube length and inversely proportional to the plate separation and the accelerating voltage. A slower electron spends longer between the plates and is deflected further, so a lower accelerating voltage gives more deflection.
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The difficulty with reducing the accelerating voltage of a cathode ray tube to gain sensitivity is that
Answer: A
The light output depends on the energy the electrons deliver to the phosphor, so a slower beam gives a fainter trace. Post-deflection acceleration resolves the conflict by accelerating the beam after it has been deflected rather than before.
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Post-deflection acceleration is applied in a cathode ray tube in order to obtain
Answer: B
A spiral or mesh electrode beyond the deflection plates raises the beam energy after the deflection has been decided, so the sensitivity corresponds to the low pre-deflection voltage while the brightness corresponds to the high final voltage.
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