VERNIER INSTRUMENT, a measuring device which enables either linear or angular magnitudes to be read with a degree of accuracy many times greater than is possible with a scale as ordinarily divided and subdivided. The principle of the vernier is readily understood from the following figure and illustration.
Let AB (see fig.) be the normal scale, i.e. a scale graduated accord ing to a standard of length, CD, a scale (placed in contact with AB for convenience) graduated so that so divisions equal II divisions of the scale AB, and EF a scale placed similarly and graduated so that so divisions equal 9 divisions of the scale AB. Consider the combina tion AB and CD. Obviously each division of CD is greater than the normal scale division. Let a represent a length to be measured, placed so that one end is at the zero of the normal scale, and the other end in contact with the end of the vernier CD marked so. It is noted that graduation 4 of the vernier coincides with a division of the stan dard, and the determination of the excess of a over 3 scale divisions reduces to the difference of 7 divisions of the normal scale and 6 di visions of the vernier. This is .4, since each vernier division equals s.s
scale divisions. Hence the scale reading of the vernier which coincides with a graduation of the normal scale gives the decimal to be added to the normal scale reading. Now consider the scales AB and EF, and let (3 be the length to be measured ; the scale EF being placed so that the zero end is in contact with an end of 0. Obviously each di vision of EF is less than that of the normal scale. It is seen that division 6 of the vernier coincides with a normal scale division, and obviously the excess of 13 over two normal scale divisions equals the difference between 6 normal scale divisions and 6 vernier divisions, i.e. o•6. Thus again in this case the vernier reading which coincides with a scale reading gives the decimal to be added to the normal scale. The second type of vernier is that more commonly adopted, and its application to special appliances is quite simple.