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A vernier is an auxiliary scale on a measuring instrument which makes it possible to accurately read fractional divisions of the main scale. An ordinary metric vernier has 10 divisions on the vernier scale covering nine divisions on the main scale.

For instance, if the smallest main scale division is 1 mm, then the vernier scale is 9 mm long and each division of the vernier scale is 0.9 mm. Namely, a vernier scale division is 0.1 mm shorter than a main scale division. Usually, divisions on the vernier scale cover (n - 1) divisions on the main scale. The difference in length between a vernier division and a main scale division is 1/n times a main scale division. This is called the least count of the instrument. For example, the least count of the device is: 1/10 x 1 mm = 0.1 mm

However, if zero on the vernier coincides with the 5 division mark on the main scale, then the first vernier mark will be 1/10 of a main scale division to the left of the main scale 6 division mark. The second vernier mark will be 2/10 to the left of the next main scale mark, and so on. Therefore, if the vernier were moved ahead so that the second vernier mark lined up with the 7 division mark on the main scale, the zero vernier mark would be 2/10 of a main scale division to the right of the 5 division mark on the main scale. The reading would then be 5.2 as illustrated in the right part of the figure.

Here are three basic steps to read a vernier:

1. Determine the least count of the instrument. This is the smallest main scale division divided by the number of vernier divisions.

2. Read the main scale division at or just to the left of the zero mark of the vernier.

3. Note which vernier division lines up with a main scale division. Multiply the number of this vernier division by the least count and add this result to the main scale reading.

For instance, if the smallest main scale division is 1 mm, then the vernier scale is 9 mm long and each division of the vernier scale is 0.9 mm. Namely, a vernier scale division is 0.1 mm shorter than a main scale division. Usually, divisions on the vernier scale cover (n - 1) divisions on the main scale. The difference in length between a vernier division and a main scale division is 1/n times a main scale division. This is called the least count of the instrument. For example, the least count of the device is: 1/10 x 1 mm = 0.1 mm

However, if zero on the vernier coincides with the 5 division mark on the main scale, then the first vernier mark will be 1/10 of a main scale division to the left of the main scale 6 division mark. The second vernier mark will be 2/10 to the left of the next main scale mark, and so on. Therefore, if the vernier were moved ahead so that the second vernier mark lined up with the 7 division mark on the main scale, the zero vernier mark would be 2/10 of a main scale division to the right of the 5 division mark on the main scale. The reading would then be 5.2 as illustrated in the right part of the figure.

Here are three basic steps to read a vernier:

1. Determine the least count of the instrument. This is the smallest main scale division divided by the number of vernier divisions.

2. Read the main scale division at or just to the left of the zero mark of the vernier.

3. Note which vernier division lines up with a main scale division. Multiply the number of this vernier division by the least count and add this result to the main scale reading.