<P> The name "vernier" was popularised by the French astronomer Jérôme Lalande (1732--1807) through his Traité d'astronomie (2 vols) (1764). </P> <P> The use of the vernier scale is shown on a vernier caliper which measures the internal and the external diameters of an object . </P> <P> The vernier scale is constructed so that it is spaced at a constant fraction of the fixed main scale . So for a vernier with a constant of 0.1, 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 . </P> <P> The difference between the value of one main scale division and the value of one Vernier scale division is known as least count of the Vernier . It is also known as Vernier constant . Let the measure of the smallest main scale reading, that is the distance between two consecutive graduations (also called its pitch) be S and the distance between two consecutive Vernier scale graduations be V such that the length of (n - 1) main scale divisions is equal to n Vernier scale divisions . Then, </P>

In a vernier micrometer vernier scale is graduated on-