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How to Read a Vernier Scale ?

Almost 220 years ago, Joseph R.Brown and his father, found ed Brown & Sharp Manufacturing Co., combined the vernier priciple with the slide caliper, creating the vernier caliper. Nowadays, vernier instruments also include height gages, depth gages, gear tooth instruments, and protractors. These simple scale instruments all use verniers to increase their amplification, which, in turn, increase their discrimination. Adding verniers to instruments that use other means of amplification, such as micrometers, increases their precision.
When a vernier scale is attached to an instrument, it slides parallel to the line of measurement, so that the main scale also slides parallel to the line of measurement. Scales are mounted so that you can make readings from one scale to the other with a minimum parallax error.
For example, we can use a vernier scale and a main scale in decimal-inches-the vernier has been a decimal instrument from its inception. Each inch is divided into 10 parts, and each division is subdivided into quarters, making the smallest division 1/40 in., or 0.025in., read as 25 mil.
The vernier scale on a caliper is attached to the sliding jaw and moves along the main scale. The inch vernier scale has 25 divisions in the same length that the main scale has 24 divisions. The difference between a main scale division and a vernier division is 1/25 of a main scale division. 1/25 of 0.025 in. equals 0.001 in. or 1 mil, which is the discrimination of the instrument or the least count. You read a vernier instrument or the controls on machines by adding the total readings on the vernier scale. Remember, only when the vernier is at zero do two lines, 0 and 25, of the vernier scale coincide with the main scale.
Take these steps to read a vernier instrument:
1. Read the number of whole divisions on the main scale that appear to the left of zero (0) in the vernier.
2. Read the largest numbered graduation on the main scale that lies to the right of the index (0) on the vernier scale. Read them as even 100 mil (0.100, 0.200, and so forth). Add to the whole reading from step 1 (1.100, 2.100, and so forth).
3. Read the largest whole minor division to the right of the index. Read these graduations in increments of 25 mil (0.000, 0.025,0.050, and 0.075in.). Add to the sum of steps 1 and 2 (1.125,1.150 in. and so forth).
4. Find the vernier graduation that most exactly coincides with any graduation on the main scale. This measurement is the nearest mil and may be any whole number from zero to 25 (0.000,0.001�.024, 0.025). Add the results to the sum of the previous three steps.