# Which of the following is the most precise device for measuring length:

Question.
Which of the following is the most precise device for measuring length:

(a) a vernier callipers with 20 divisions on the sliding scale

(b) a screw gauge of pitch 1 mm and 100 divisions on the circular scale

(c) an optical instrument that can measure length to within a wavelength of light ?

solution:

A device with minimum count is the most suitable to measure length.

(a) Least count of vernier callipers

$=1$ standard division $(S D)-1$ vernier division $(V D)$

$=1-\frac{9}{10}=\frac{1}{10}=0.01 \mathrm{~cm}$

(b) Least count of screw gauge $=\frac{\text { Pitch }}{\text { Number of divisions }}$

$=\frac{1}{1000}=0.001 \mathrm{~cm}$

(c) Least count of an optical device $=$ Wavelength of light $\sim 10^{-5} \mathrm{~cm}$

$=0.00001 \mathrm{~cm}$

Hence, it can be inferred that an optical instrument is the most suitable device to measure length.