In the context of vernier calliper, an internal jaw is used to measure:

Correct — D. A vernier calliper carries two pairs of jaws pointing in opposite directions. The lower, external jaws close on an object from outside. The upper, internal jaws open outwards from inside a hollow object.

So the internal jaws are pushed into a pipe or a hollow cylinder and opened until they touch the wall, and the scale then reads the internal diameter — option (d).

The thin strip that slides out of the back of the main scale is a third part, the depth gauge.

Why the others are wrong

• (a)
  the depth of a beaker — Depth is read by the depth rod, the thin strip that extends from the end of the main scale as the jaws open. That is a different part of the instrument.
• (b)
  the length correct up to 1 mm — An ordinary metre scale already reads to 1 mm. The point of the vernier is that it resolves a fraction of the smallest main-scale division, so this understates the instrument rather than describing a jaw.
• (c)
  the length of a rod and diameter of a sphere — A rod's length and a sphere's diameter are external dimensions, gripped by the lower external jaws. Right instrument, wrong pair of jaws.

A vernier calliper is a main scale with a sliding vernier scale beside it. The vernier divisions are slightly shorter than the main-scale divisions, so exactly one vernier mark lines up with a main-scale mark, and which one lines up gives the fraction.

Least count = one main-scale division minus one vernier-scale division. A common school instrument has 10 vernier divisions spanning 9 mm, giving a least count of 0.1 mm.

The instrument has three measuring parts, each with one job: external jaws, internal jaws and the depth rod. Questions test which part does which.

Key facts

• The internal, upper jaws of a vernier calliper measure internal diameters, such as the bore of a pipe.
• The external, lower jaws measure external dimensions, such as the diameter of a sphere.
• The depth rod at the end of the main scale measures the depth of a hole or a beaker.
• Least count is one main-scale division minus one vernier-scale division, commonly 0.1 mm on a school calliper.

Common traps

• Choosing the rod-and-sphere option because it sounds like the calliper's main use — it is, but with the other pair of jaws
• Treating 1 mm as the calliper's precision when 1 mm is what a plain scale already gives

No directly related past PYQ was found.