This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.

An important measuring instrument used in machine operation is the vernier (Fig. 201), so called from the inventor's name.

It consists of a bar of metal divided into inches, each inch being again divided into ten parts, and each tenth into four parts, making forty parts to the inch. On the sliding jaw is a line of division called the vernier which consists of 25 parts, numbered 0, 5, 10, 15, 25. The 25 parts on the vernier correspond, in extreme length, with 24 parts or 24/40 of an inch on the bar. Consequently each division on the vernier is smaller than each division on the bar by one-thousandth of an inch.

If the sliding jaw of the caliper is pushed along the other, so that the line marked 0 on the vernier corresponds with that marked 0 on the bar, then the two next lines to the right will differ from each other by one-thousandth of an inch. The difference continues to increase one-thousandth of an inch for each division, till they again correspond at the line marked 25 on the vernier. To read the distance when the caliper is open, we begin by noticing how many inches, tenths, and parts of tenths the zero point on the vernier has been moved from the zero point on the bar. We now count upon the vernier the number of divisions, until one is found which coincides with one on the bar. This division will be the number of thousandths to be added to the distance read off the bar. The best way of expressing the value of the divisions on the bar is to call the tenths, one hundred times one thousandth (.100), and the fourths of tenths, or fortieths, twenty-five times one thousandth (.025).

The vernier shown in Fig. 201 has been moved to the right, 1 2/10 in., or 1.2 in., as indicated by the bar; and the sixth line on the vernier coincides with a line on the bar, thus making six-thousandths (.006) of an inch to be added to the reading from the scale, which would make the total reading one and two hundred and six thousandths (1.206) inch.

In making inside measurements with a 6 in. vernier, two and one-half tenths, or two hundred and fifty thousandths (.250), of an inch and with the 12 in. and 24 in. vernier, three-tenths, or three hundred times one thousandth (.300), of an inch, should be added to the apparent reading on the vernier side for the space occupied by the caliper points. With a vernier caliper (Fig. 202) reading to metric measure, add 6 mm. When the other side of the instrument is used no deduction is necessary, as there are two lines, one indicating inside and the other outside measurements.

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