Least count is the smallest measurement that an instrument can measure accurately. It tells us the precision of the measuring instrument.
An instrument with smaller least count gives more precise measurements. Before using any measuring instrument, its least count must be known.
A metre scale is used to measure length. The least count of a metre scale is usually 1 mm (0.1 cm).
Vernier calipers are used to measure small lengths, internal and external diameters, and depths.
Least count = Value of one main scale division – Value of one vernier scale division
A screw gauge is used to measure very small lengths like diameter of a wire or thickness of a sheet.
Least count = Pitch / Number of divisions on circular scale
If an instrument does not show zero reading when the measured quantity is zero, it has zero error.
Q1. Least count of an instrument represents: A) Accuracy B) Precision C) Range D) Error Answer: B Q2. Least count of a metre scale is usually: A) 1 cm B) 0.1 cm C) 0.01 cm D) 1 m Answer: B Q3. Which instrument is used to measure the diameter of a thin wire? A) Metre scale B) Vernier calipers C) Screw gauge D) Measuring tape Answer: C Q4. Least count of a screw gauge depends on: A) Radius B) Length C) Pitch and circular scale divisions D) Diameter Answer: C Q5. Zero error occurs when: A) Instrument is perfectly adjusted B) Reading is maximum C) Instrument does not read zero at zero measurement D) Least count is zero Answer: C
Smallest value that an instrument can measure. Smaller least count → higher precision.
Always apply zero correction after finding the observed reading.
