Which of the following instruments is best for measuring the diameter of a thin constantan wire?

Step 1: Analyze the question You need the instrument that measures a very small length, the diameter of a thin wire, with the greatest precision.

Step 2: Concept: precision of measuring instruments

| Instrument | Typical precision | |---|---| | Metre rule | 0.1 cm (1 mm) | | Callipers (ordinary) | about 1 mm or worse, used only for rough comparison | | Vernier callipers | 0.01 cm (0.1 mm) | | Micrometer screw gauge | 0.001 cm (0.01 mm) |

A thin wire usually has a diameter of a fraction of a millimetre (for example 0.20 mm). To measure it with a small percentage error, you need the finest instrument available.

Step 3: Evaluate each option

• A. Callipers: These are not precise enough and only transfer a measurement to another scale.
• B. Metre rule: It reads only to 1 mm, which is too coarse for a wire under 1 mm thick.
• C. Micrometer screw gauge: It reads to 0.01 mm, which makes it ideal for thin wires.
• D. Vernier callipers: They read to 0.1 mm. That is better than a rule but still not fine enough for a thin wire.

Step 4: Answer The micrometer screw gauge is the best choice.

Tip: Match the instrument to the size of the object.

• Longer lengths: metre rule or tape
• Small objects such as the diameter of a ball, or the internal diameter of a tube: vernier callipers
• Very small objects such as wires, hair, or the thickness of paper: micrometer screw gauge

Also remember to check for zero error on the micrometer before you use it.