JEE Main · 2020 · 06 Sep 2020 · Shift 1 · Units & Measurement

One full turn of the circular scale advances the spindle by the pitch, $0.5\text{mm}$, and this is divided into $50$ equal parts, so the least count is $\frac{0.5}{50}=0.01\text{mm}=10\mu \text{m}$. Since the circular scale reads four divisions ahead of the reference mark before any object is gripped, the instrument reads more than the true value, which is a positive zero error. The error is positive and the least count is $10\mu \text{m}$.