In a colour picture tube (CRT) used in television, what does “degaussing” refer to and why is it performed?

Introduction / Context:
Colour CRTs employ a shadow mask and phosphor triads. Residual magnetization of metallic parts (mask, cabinet shielding) distorts electron beam landing, causing colour purity errors and blotches. Degaussing corrects this by demagnetizing ferromagnetic components near the display area.

Given Data / Assumptions:

• Colour CRT with a shadow mask.
• Presence of residual magnetism from transport, nearby speakers, or Earth's field exposure.
• Degaussing coil present around the CRT face.

Concept / Approach:

Degaussing applies a decaying alternating magnetic field that randomizes magnetic domains, driving the net magnetization toward zero. This restores correct beam landing on red, green, and blue phosphors, improving colour purity. It does not increase or decrease coil resistance for time constants; such options confuse power/deflection circuit design with degaussing function.

Step-by-Step Solution:

Verification / Alternative check:

TV service practice: degaussing thermistor circuits or manual degaussing wands are used after relocation or magnet exposure to restore picture quality.

Why Other Options Are Wrong:

• Adding magnetism: opposite of the required action.
• Altering coil resistance/time constant: not the purpose of degaussing; that relates to different circuits.
• Mechanical alignment: relates to focus/convergence, not demagnetization.

Common Pitfalls:

Confusing convergence with purity; degaussing fixes magnetic contamination, while convergence adjusts beam geometry/electromagnetic alignment.

Final Answer:

Removing residual magnetism from the shadow mask, shield, and nearby metal parts