The inner diameter of a small test tube can be measured accurately using a

Analysis

The question asks for the most appropriate instrument to measure the inner diameter of a small test tube. To solve this, we must consider the precision required and the physical design of the measuring tools.

Concept Explanation

In physics, different measuring instruments are designed for specific tasks based on their precision and their physical jaws:

1. Metre rule: Used for general length measurements. It lacks the precision for small diameters and cannot reach inside a narrow tube.
2. Pair of dividers: Used for transferring distances or comparing lengths, not for direct measurement of diameters.
3. Micrometer screw gauge: Highly precise for measuring external dimensions (like the thickness of a wire or a thin sheet), but its spindle cannot reach inside a test tube to measure an internal diameter.
4. Vernier calliper: This instrument is versatile. It has two sets of jaws: • External jaws: For measuring the outside diameter or thickness of an object. • Internal jaws (the upper, smaller prongs): Specifically designed to be inserted into a hole or a tube to measure the internal diameter accurately.

Step-by-Step Solution

1. Identify the requirement: We need to measure an internal dimension of a small object (the test tube).
2. Evaluate the tools: • A metre rule is too bulky and inaccurate for this. • A micrometer screw gauge is designed for external measurements only. • A pair of dividers is not a measuring scale.
3. Select the tool: The Vernier calliper is the only instrument listed that features specialized internal jaws designed to fit inside a tube to provide an accurate reading of the inner diameter.

Conclusion

The correct instrument for measuring the inner diameter of a test tube is the Vernier calliper.

Exam Tip

Remember the "Jaws" rule:

• External Jaws (bottom) = Outside diameter/thickness.
• Internal Jaws (top) = Inside diameter/width.
• Tail/Depth Probe (the thin rod that slides out) = Depth of a hole or test tube.