A vernier scale is a fine auxiliary scale set alongside the main scale of a measuring instrument, letting you read a fraction of one main-scale division accurately. It is best known from the vernier caliper, but also appears on protractors, theodolites and levelling instruments. In Dutch and German the same scale is called a nonius.
How it works
The trick lies in the difference in division spacing. On a caliper reading to 0.05 mm, twenty vernier divisions are spread over exactly 39 mm of the main scale. Each vernier mark therefore sits slightly out of step with the main scale. The one vernier mark that lines up perfectly with a main-scale mark tells you the decimal.
Reading it in three steps
- Read the whole millimetres — see where the vernier zero sits on the main scale and read the last millimetre passed
- Find the aligned mark — run along the vernier until one mark lines up exactly with a main-scale mark
- Add them up — the number of that mark gives the decimals; 12 mm plus mark 7 on a 0.05 vernier gives 12.35 mm
Accuracy
| Vernier | Reading | Use |
|---|---|---|
| 10 divisions | 0.1 mm | Basic calipers |
| 20 divisions | 0.05 mm | Standard workshop caliper |
| 50 divisions | 0.02 mm | Finer metalwork |
For finer work still you use a micrometer rather than a vernier.
Name and origin
The Dutch name nonius refers to the Portuguese mathematician Pedro Nunes (Latinised: Nonius), who devised a related device in the sixteenth century. The scale as we know it today was developed by the Frenchman Pierre Vernier in the seventeenth century — hence the English name.
Practical tips
- Clean the jaws and the scale before measuring; a single speck of dust is worth several hundredths
- Check the zero: close the jaws and see that zero lines up with zero
- Always read square-on to the scale, or parallax will make you read the wrong mark
- A digital caliper is quicker, but a vernier works without a battery and is therefore always ready
Related terms
- Caliper
- Micrometer
- Setting out
- Levelling instrument
- Measuring tape
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