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Physics · Chapter 2: Physical Quantities and Measurement
Screw Gauge
Parts of a micrometer screw gauge, pitch and least count 0.01 mm, taking a reading, the ratchet and zero error.
Key termRememberExampleWatch out
01Concept
To measure the thickness of a wire, a sheet of paper or a hair, we need an even more precise instrument: the screw gauge (micrometer screw gauge). Its least count is 0.01 mm, ten times smaller than vernier callipers.
Parts
- U-shaped frame that holds everything.
- Anvil: the fixed stud. Spindle: the moving rod. The object is held between them.
- Sleeve with the main (linear) scale in millimetres along a reference line.
- Thimble with the circular scale, usually 100 (or 50) equal divisions.
- Ratchet: turns the thimble but slips once the object is held lightly, so you do not press too hard.
Pitch and least count
The pitch is the distance the spindle moves forward in one complete turn of the thimble. In most school screw gauges the pitch is 1 mm with 100 circular divisions, or 0.5 mm with 50 divisions.
Least count = pitch ÷ number of divisions on the circular scale = 1 mm ÷ 100 = 0.01 mm
Taking a reading
- Turn the ratchet until the object is held gently between the anvil and spindle.
- Read the main scale reading (MSR): the last mark visible on the sleeve. 5 mm
- Read the circular scale division that lines up with the reference line. 38
- Circular scale reading = 38 × 0.01 = 0.38 mm
- Total = MSR + circular scale reading = 5 + 0.38 = 5.38 mm
Zero error
When the anvil and spindle touch, the zero of the circular scale should be on the reference line. If not, there is a zero error.
| Positive zero error | Negative zero error | |
|---|---|---|
| Circular scale zero (jaws closed) | Below the reference line | Above the reference line |
| Effect | Readings too large | Readings too small |
Corrected reading = observed reading − zero error, exactly as for vernier callipers.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Key Formulas
Least count of a screw gauge
1 mm ÷ 100 = 0.01 mm, or 0.5 mm ÷ 50 = 0.01 mm.
Screw gauge reading
- = circular scale division on the reference line
04Real World Examples
Electricians
The thickness of copper wire decides how much current it can safely carry. Wire factories check diameters with micrometer screw gauges.
Paper and plastic
Paper mills and plastic-bag factories in Pakistan measure sheet thickness in microns (µm) with micrometers.
Engine parts
Engine pistons and crankshafts are measured with micrometers to 0.01 mm, because a small difference can ruin the engine.
05Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define pitch of a screw gauge.Show answer
Answer
Pitch is the distance moved by the spindle in one complete rotation of the thimble.
Q2.Find the least count of a screw gauge with a pitch of 0.5 mm and 50 circular divisions.Show answer
Answer
- LC = pitch ÷ number of circular divisions
- = 0.5 mm ÷ 50
- = 0.01 mm
Q3.The main scale reading is 3 mm and the 46th circular division is on the reference line. Find the reading (LC = 0.01 mm).Show answer
Answer
- Circular reading = 46 × 0.01 = 0.46 mm
- Total = 3 + 0.46 = 3.46 mm
Q4.What is the use of the ratchet?Show answer
Answer
The ratchet slips when the object is held with the right pressure, so the object is not squeezed and the screw is not damaged. This gives the same pressure every time.
Q5.A screw gauge has a zero error of +0.03 mm. A wire reads 1.27 mm. Find its true diameter.Show answer
Answer
- Corrected = 1.27 − 0.03
- = 1.24 mm
Long questions
Q6.Describe a screw gauge and explain how you would use it to find the diameter of a wire.Show answer
Answer
- Find the pitch (e.g. 1 mm) and count the circular divisions (e.g. 100). LC = 1 ÷ 100 = 0.01 mm.
- Close the gauge with the ratchet and note the zero error with its sign.
- Place the wire between the anvil and spindle and turn the ratchet until it clicks.
- Read the main scale: e.g. 0 mm (or 0.5 mm if a half-mark is visible).
- Read the circular scale division on the reference line: e.g. 52 → 0.52 mm.
- Observed = MSR + circular reading = 0.52 mm.
- Corrected = observed − zero error.
- Repeat at three places along the wire, in two directions, and take the mean.