A Vernier scale is a device that lets the user measure more precisely than could be done unaided when reading a uniformly-divided straight or circular measurement scale. It is scale that indicates where the measurement lies in between two of the marks on the main scale.
The main use of the Vernier caliper is to measure the internal and the external diameters of an object. To measure using a Vernier scale, the user first reads the finely marked "fixed" scale (in the diagram). This measure is typically between two of the scale's smallest graduations. The user then reads the finer Vernier scale (see diagram), which measures between the smallest graduations on the fixed scale—providing much greater accuracy.
The Vernier scale is constructed so that it is spaced at a constant fraction of the fixed main scale. So for a decimal measuring device each mark on the Vernier is spaced nine tenths of those on the main scale. If you put the two scales together with zero points aligned, the first mark on the vernier scale is one tenth short of the first main scale mark, the second two tenths short, and so on up to the ninth mark—which is misaligned by nine tenths. Only when a full ten marks are counted is there alignment, because the tenth mark is ten tenths—a whole main scale unit short, and therefore aligns with the ninth mark on the main scale.
Now if you move the Vernier by a small amount, say, one tenth of its fixed main scale, the only pair of marks that come into alignment are the first pair, since these were the only ones originally misaligned by one tenth. If we move it two tenths, the second pair aligns, since these are the only ones originally misaligned by that amount. If we move it five tenths, the fifth pair aligns—and so on. For any movement, only one pair of marks aligns and that pair shows the value between the marks on the fixed scale.
Source: Wikipedia - Vernier Scale. Click the link to read the entire Wikipedia entry.
This week, you'll find many activities and simulations that will help you learn to accurately read the Vernier Caliper. Many of the activities are available to you in part thanks to the contributions of Professor Eduardo J. Stefanelli @ www.stefanelli.eng.br.