One of the following readings represents the measurement of the length of a metal rod using vernier callipers…

Analysis

To solve this question, we need to understand the precision and least count of a standard vernier calliper.

Concept: Least Count and Precision

1. What is a Vernier Calliper? It is a precision instrument used to measure lengths with a higher degree of accuracy than a standard meter rule.
2. The Least Count: The least count of a standard vernier calliper is typically 0.01 cm (or 0.1 mm). This means the instrument is designed to measure accurately up to two decimal places in centimeters.
3. Reading Accuracy: When using a vernier calliper, the final digit is often an estimate based on the alignment of the vernier scale with the main scale. Because the instrument's precision is limited to 0.01 cm, any reading provided must reflect this limit.

Step-by-Step Evaluation

• Option C (5.0 cm): This is the precision of a standard meter rule. It is too crude for a vernier calliper.
• Option B (5.165 cm): This measurement has three decimal places. A standard vernier calliper cannot measure to the third decimal place (thousandths of a centimeter); that level of precision would require a micrometer screw gauge.
• Option D (5.160 cm): While this has two decimal places, the "0" at the end implies a level of precision that is technically redundant for a standard vernier calliper, which reads to the nearest 0.01 cm.
• Option A (5.16 cm): This reading is expressed to two decimal places (0.01 cm). This perfectly matches the least count and the standard reporting format for a vernier calliper.

Conclusion

Since the vernier calliper has a least count of 0.01 cm, the most appropriate reading must be recorded to two decimal places. Option A is the only value that correctly represents the precision capability of the instrument.

Exam Tip: Always check the number of decimal places in your measurement instruments!

• Meter rule: 1 decimal place (e.g., 5.1 cm)
• Vernier calliper: 2 decimal places (e.g., 5.16 cm)
• Micrometer screw gauge: 3 decimal places (e.g., 5.165 cm)