I found an answer from ncert.nic.in

Experiments 1-5.pmd

(iii) **measure** the internal diameter and depth of a given cylindrical object ... and
one **vernier scale** division (V.S.D.) is called the **least count** of the ... scale. If this is
not so, account for the zero **error** for all observations to ... Express the result in
suitable **units** ... to 0.01 mm or 0.005 mm, may be made by using a **screw gauge**.
As.

For more information, see Experiments 1-5.pmd

I found an answer from en.wikipedia.org

Micrometer - Wikipedia

A micrometer, sometimes known as a micrometer **screw gauge**, is a **device**
incorporating a calibrated screw widely used for **accurate measurement** of ...

For more information, see Micrometer - Wikipedia

I found an answer from www.quora.com

What is the **least** count of a spectrometer? - Quora

A spectrometer is always provided with a **circular scale** for **measuring** angles of
rotation. However, for **accurate measurement** of angles, there is a **vernier scale** ...

For more information, see What is the **least** count of a spectrometer? - Quora

a)

The Vernier calliper least counts formula is calculated by dividing the smallest main scale reading by the total number of vernier scale divisions.

Given that

Divisions in vernier callipers = 20

Smallest main scale reading = 1 mm

Least count of Vernier calliper = \frac{1}{20} mm

= 0.05 mm = 0.005 cm

b)

Screw gauge least count = \frac{ \text{ screw pitch }}{ \text{ number of circular scale divisions } }

Given that

screw pitch = 1 mm

number of circular scale divisions = 100

Screw gauge least count = \frac{1}{100} mm = 0.01 mm = 0.001 cm

c)

Least count of the optical instrument = wavelength of the light

wavelength of the light = 10^{-5} cm

= 0.00001 cm.

Of course, optical instrument is the most suitable instrument for accurate measurement.