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

The correct answer is the internal diameter of a hollow cylinder and pipes.

Let us understand the construction and functioning of a Vernier caliper to see why this is the correct choice:

1. Main Components of a Vernier Caliper:
A standard Vernier caliper consists of two sets of jaws and a depth probe:
- External Jaws (Lower Jaws): These are larger jaws used to measure external dimensions, such as the length of a rod, width of an object, or the outer diameter of a sphere or cylinder.
- Internal Jaws (Upper Jaws): These are smaller jaws pointing outwards. They are specifically designed to fit inside hollow spaces to measure internal dimensions.
- Depth Stretcher/Probe (Stem): A thin strip at the rear end of the caliper used to measure the depth of vessels, beakers, or holes.

2. Function of the Internal Jaw:
When we insert the upper internal jaws inside a hollow object (like a pipe, tube, or hollow cylinder) and open them until they touch the inner walls, the scale gives the measurement of the inner distance between those walls. Therefore, the internal jaw is used to measure the internal diameter of a hollow cylinder and pipes.