Vernier scales are used in a number of applications, but you will probably come across them in the lab when using callipers, angular vernier scales and travelling microscopes. Vernier Scales allow you to read off a value more precisely than when using an ordinary scale.
Fig. 1: Vernier callipers, marked with a Vernier scale
Fig. 2: Detail of Vernier scale.
Fig. 3: The small scale slides along the main scale.
The Vernier scale, which runs next to the main scale, is designed so that ten of its division have the same length as nine of the divisions on the main scale.
In figure 2 the markings on the main scale are at (say) 1mm intervals, while those on the Vernier scale are at 0.9mm intervals. The difference between these sizes tells us what precision the Vernier scale will measure to, in this case 0.1mm. When you take a reading, the Vernier scale will slide along the main scale as shown in figure 3.
Images adapted from The Open Door Web Site
To take a reading:
You can see why this works in figure 4. Two ticks on the Vernier scale represents 1.8mm while two ticks on the main scale represents 2.0mm. Hence the difference, which is the reading we want, is 0.2mm.
Fig. 4: Vernier Scale
To have a go at reading a vernier calliper, see our vernier calliper interactive experiment (link and content needed).