The most suitable instrument for measuring the diameter of a thin wire is a

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

2. Explain the concept Each instrument has a different smallest reading (its precision):

| Instrument | Typical precision | |---|---| | Meter rule | 1 mm (0.1 cm) | | Pair of dividers | Only transfers lengths and has no scale of its own | | Engineer's calipers | About 0.5 to 1 mm | | Vernier calipers | 0.1 mm (0.01 cm) | | Micrometer screw gauge | 0.01 mm (0.001 cm) |

A thin wire usually has a diameter of less than 1 mm, so we need an instrument that reads to 0.01 mm.

3. Step-by-step solution

• Meter rule (A): Too coarse, because it reads only to the nearest mm. A wire 0.5 mm thick would carry a large percentage error.
• Dividers (C): These only transfer a length and do not measure it directly.
• Engineer's calipers (D): These are used for rough measurements of larger objects such as pipes and rods.
• Vernier calipers (E): These are better, at 0.1 mm precision, but still not fine enough for a very thin wire.
• Micrometer screw gauge (B): It works by turning a screw with a very fine pitch (usually 0.5 mm per turn). The thimble is divided into 50 or 100 parts, which gives a reading to 0.01 mm. This makes it the best choice for thin wires.

4. Answer The correct option is B. micrometer screw gauge.

5. Exam tip Remember the ordering from coarse to fine: meter rule → vernier calipers → micrometer screw gauge. If the question mentions a thin wire, a hair, or a small sphere, think "micrometer screw gauge." If it mentions a depth, an internal diameter, or a medium-sized object, think "vernier calipers."

Also, always check for zero error on a micrometer before use and correct your final reading for it.