Troubleshooting input devices: if a traditional mechanical (ball) mouse pointer moves erratically on the screen, which underlying cause is most likely?

Introduction / Context:
Before optical sensors became standard, mice used a rubber ball to translate motion into two perpendicular rollers. Erratic or jumpy cursor behavior is a common troubleshooting scenario in labs and support desks, and recognizing telltale symptoms saves time.

Given Data / Assumptions:

• The device is explicitly behaving erratically (jitter, sticking, sudden jumps).
• We are considering a mechanical ball mouse, not a modern optical/laser mouse.
• No mention of complete non-function (which would suggest power, port, or driver failures).

Concept / Approach:

Mechanical mice rely on friction between the desk surface, the rubber ball, and internal rollers. Dust and skin oils accumulate on the ball and rollers, creating slippage and intermittent contact. This contamination introduces uneven rotation detection, which appears as jittery motion. Cleaning the ball and rollers restores smooth tracking.

Step-by-Step Solution:

Verification / Alternative check:

Support playbooks recommend removing the retaining ring, cleaning the ball with isopropyl alcohol, and gently scraping residue off rollers. Immediate improvement confirms the diagnosis.

Why Other Options Are Wrong:

• Not connected: would yield no movement at all, not erratic tracking.
• Driver not installed properly: often results in non-operation or generic behavior, not sporadic jitter specific to movement.
• Incorrect IRQ: was an issue for legacy ISA devices but rarely manifests as subtle jitter; it typically prevents correct operation altogether.

Common Pitfalls:

• Replacing the mouse before attempting simple cleaning.
• Ignoring the desk surface; very glossy surfaces can also cause problems, but dirt remains the top cause for ball mice.

Final Answer:

The mouse ball is dirty.