A vernier caliper is a precision measuring instrument that reads lengths, inside diameters, outside diameters and depths, usually to 0.02 mm. It has a fixed main scale and a sliding vernier scale; the small offset between the two scales lets you read a fraction of the smallest main-scale division. Its main parts are the main scale, vernier scale, outside jaws, inside jaws, depth rod, thumb screw and locking screw.
Vernier caliper diagram: parts and their functions
| Part | What it does |
|---|---|
| Main scale | The fixed beam graduated in millimetres (and often inches on the other edge). |
| Vernier scale | The sliding scale that gives the fraction of a millimetre. |
| Outside (external) jaws | The large lower jaws that grip the outside of an object to measure length, thickness or outer diameter. |
| Inside (internal) jaws | The small upper jaws that open against the walls of a hole to measure inside diameter or slot width. |
| Depth rod (depth probe) | A thin rod that slides out of the end of the beam to measure the depth of holes and steps. |
| Thumb screw / roller | Lets you move the vernier smoothly with your thumb. |
| Locking screw | Fixes the vernier in place so the reading does not change while you read it. |
Least count of a vernier caliper
The least count is the smallest length the instrument can measure:
Least count = 1 main scale division (MSD) − 1 vernier scale division (VSD) = value of 1 MSD ÷ number of vernier divisions
| Vernier divisions | They cover | Least count |
|---|---|---|
| 10 | 9 mm | 1 mm ÷ 10 = 0.1 mm |
| 20 | 19 mm (or 39 mm) | 0.05 mm |
| 50 | 49 mm | 0.02 mm |
Most workshop and lab calipers have 50 vernier divisions and a least count of 0.02 mm. School physics lab calipers often have 10 divisions and read to 0.1 mm.
How to take a reading
- Close the jaws fully and check for zero error (see below).
- Place the object between the correct jaws and close them gently. Do not squeeze.
- Tighten the locking screw and remove the caliper if needed.
- Main scale reading (MSR): read the main-scale mark just to the left of the vernier zero.
- Vernier coincidence (VC): find the vernier line that lines up exactly with any main-scale line.
- Total reading = MSR + (VC × least count) − zero error.
Example. Least count 0.02 mm. The vernier zero is just past 24 mm, and the 17th vernier line coincides with a main-scale line. Reading = 24 + 17 × 0.02 = 24 + 0.34 = 24.34 mm.
Zero error and its correction
- Positive zero error: with the jaws closed, the vernier zero sits to the right of the main-scale zero. Subtract the error from every reading.
- Negative zero error: the vernier zero sits to the left of the main-scale zero. The correction is added. For a negative error, count the coinciding division from the far end of the vernier: error = −(n − VC) × LC.
For example, with a positive zero error of +0.04 mm, a reading of 24.34 mm corrects to 24.30 mm.
Types of vernier caliper
| Type | How you read it | Typical resolution | Good for |
|---|---|---|---|
| Vernier (analogue) | Line up the two scales | 0.02 mm | Labs, exams, workshops; no battery needed |
| Dial caliper | A needle on a round dial shows the fraction | 0.02 mm (or 0.01 mm) | Faster reading, fewer reading errors |
| Digital caliper | LCD display; zero can be set anywhere | 0.01 mm | Quick comparison measurements and mm/inch switching |
Uses of a vernier caliper
- Measuring the diameter of shafts, rods, wires and bolts in workshops.
- Checking the bore of pipes, bearings and holes with the inside jaws.
- Measuring the depth of blind holes, keyways and steps with the depth rod.
- Quality checks on machined parts against drawing tolerances.
- Physics practicals such as finding the volume of a cylinder or sphere.
Care and accuracy tips
- Keep the jaws clean and never use the caliper as a scriber, spanner or clamp.
- Read the scale straight on to avoid parallax error.
- Measure at room temperature; steel expands about 0.012 mm per metre per °C.
- For accuracy better than 0.01 mm, use a micrometer instead.
Frequently asked questions
What is the least count of a vernier caliper?
Most vernier calipers have a least count of 0.02 mm. School lab calipers with 10 vernier divisions have a least count of 0.1 mm.
What are the main parts of a vernier caliper?
Main scale, vernier scale, outside jaws, inside jaws, depth rod, thumb screw and locking screw.
How do you calculate a vernier caliper reading?
Reading = main scale reading + (coinciding vernier division × least count), then correct for any zero error.
What is the difference between a vernier caliper and a micrometer?
A vernier caliper measures outside, inside and depth dimensions to about 0.02 mm. A micrometer measures mainly outside dimensions, but more precisely, typically to 0.01 mm or better.
References
- NCERT Class 11 Physics Lab Manual, experiment on the vernier callipers.
- NIST – Dimensional metrology