Working Principle: Rack and Pinion

The question asks which instrument operates based on the rack and pinion mechanism. A rack and pinion is a type of gear arrangement where a circular gear (pinion) meshes with a linear gear (rack). This converts linear motion to rotational motion or vice versa.

Instrument Analysis

• Vernier bevel protractor: Used for measuring angles. Its operation relies on vernier scales for precise readings, not a rack and pinion system.
• Dial test indicator: This instrument measures small deviations or displacements. It typically uses a lever mechanism connected to a rack, which engages with a pinion connected to the indicator's pointer. This setup translates the small linear movement of the contact point into a magnified rotational movement displayed on the dial, directly utilizing the rack and pinion principle.
• Vernier height gauge: Used for marking and measuring height from a base. It employs a vernier scale and a sliding jaw mechanism, but does not primarily use a rack and pinion for its core measurement function.
• Micrometer: Measures small distances with high precision using a calibrated screw thread mechanism, not a rack and pinion.

Conclusion

Based on the analysis, the Dial test indicator is the instrument that works on the principle of rack and pinion.