Calipers
Vernier caliper

The labelled parts are

The calipers in the diagram show a primary reading on the metric scale of about 2.475 cm (2.4 cm read from the main scale plus about 0.075 cm from the vernier scale).

Calipers often have a "zero point error": meaning that the calipers do not read 0.000 cm when the jaws are closed. The zero point error must always be subtracted from the primary reading. Let us assume these calipers have a zero-point error of 0.013 cm. This would give us a length reading of 2.462 cm.

For any measurement, reporting the error on the measurement is also important. Ignoring the possibility of interpolation by eye, both the primary reading and the zero point reading are bounded by plus/minus half the length corresponding to the width of the smallest interval on the vernier scale (0.0025 cm). These are "absolute" errors and absolute errors add, so the length reading is then bounded byplus/minus the length corresponding to the full width of the smallest interval on the vernier scale (0.005 cm). Assuming no systematics affect the measurement (the instrument works perfectly), a complete measurement would then read 2.462 cm ± 0.005 cm.

The vernier, dial, and digital calipers directly read the distance measured with high accuracy and precision. They are functionally identical, with different ways of reading the result. These calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types.

The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simply calibrated caliper, but the addition of a vernier scale allows more accurate interpolation and is the universal practice; this is the vernier caliper.

Vernier, dial, and digital calipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and in many cases depth by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools.

The vernier scales may include metric measurements on the lower part of the scale and inch measurements on the upper, or vice versa, in countries that use inches. Vernier calipers commonly used in industry provide a precision to 0.01 mm (10 micrometres), or one thousandth of an inch. They are available in sizes that can measure up to 1828 mm (72 in).
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