Key relationship
Least count = pitch / thimble divisions. Reading = sleeve scale + thimble division × least count − zero error.
Physics · Practical apparatus
Screw gauge
A screw advances by its pitch in one full turn. Dividing that turn into many thimble marks gives a small least count. Include visible half-millimetre marks on the sleeve before adding the thimble reading. Correct a known zero error with its sign.
Open the interactive experiment →Turn rotation into a small, measurable distance.
திருகு அளவியின் ஒரு முழுச் சுற்றில் நகரும் தூரம் திருகுப் புரியிடைத் தூரம். அதை வட்ட அளவுப் பிரிவுகளால் வகுத்தால் மீச்சிற்றளவு கிடைக்கும். பூஜ்யப் பிழையைக் கழிக்க வேண்டும்.
Key notes, relationships and model limits
Practical tip
A screw advances by its pitch in one full turn. Dividing that turn into many thimble marks gives a small least count. Include visible half-millimetre marks on the sleeve before adding the thimble reading. Correct a known zero error with its sign.
Common trap
Ignoring a visible half-millimetre sleeve mark causes an error far larger than the least count.
Model limits
Ideal 0.5 mm pitch screw. Expanded thimble is unwrapped for reading; its markings wrap at a full turn. The sleeve includes half-millimetre divisions. Contact force, backlash, elastic deformation and ratchet mechanics are not simulated.
What you will learn
Calculate pitch and least count; read half-millimetre sleeve divisions; apply zero correction.
Try the experiment
Adjust the diameter and read the unwrapped thimble. Compare 50 and 100 divisions and test a negative zero error.
Think of it this way
A spiral staircase converts one turn into a fixed vertical rise; the screw converts a turn into axial distance.
Worked example
Sleeve 5.0 mm and thimble division 37 give 5.0 + 37 × 0.01 = 5.37 mm. With −0.03 mm zero error, add 0.03 mm to the indicated value.
Why this matters
A micrometer helps measure wire diameter; diameter uncertainty strongly affects area because A is proportional to d².
Before you start
Least count · division · signed corrections
Check your understanding
A pitch of 0.5 mm and 50 thimble divisions gives which least count?
- 0.1 mm
- 0.01 mm
- 0.5 mm
Read the explanation
Least count = 0.5 / 50 = 0.01 mm.
Course connection: Practical apparatus preparation: CBSE XI, NEET/Main experimental skills and JEE Advanced General. Physical apparatus practice remains necessary.