To find the thickness of a rectangular glass slab with a slide calipers, the main-scale reading is 18 mm and the vernier coincidence is 8. If the vernier constant of the instrument is 0.01 cm, what is the thickness?

Correct answer: d 18.8 mm

The constant 0.01 cm = 0.1 mm; thickness = 18 mm + 8 × 0.1 mm = 18.8 mm.

Why the other options are wrong

• a 8.1 mm: 8.1 mm is wrong; the main-scale reading 18 mm is missing, and 8 × 0.1 mm = 0.8 mm.
• b 8.18 mm: 8.18 mm is wrong; the thickness exceeds 18 mm because the main-scale reading alone is 18 mm.
• c 18.08 mm: 18.08 mm is wrong; the constant 0.01 cm = 0.1 mm, not 0.01 mm, so 8 × 0.1 = 0.8 mm is added.