A caliper is defined as a device that is used to measure the distance between two opposing sides of an object.  The tips of the caliper are adjusted to fit across the points to be measured, the caliper is then removed and the distance is read by measuring between the tips with a measuring tool, such as a ruler.

A caliper is used in many fields such as metalworking, woodworking, and mechanical engineering.  It must be properly applied against the part being measured in order to take the desired measurement.  For example, when measuring the thickness of a plate a vernier caliper must be held at right angles to the piece.  Some practice may be needed to measure round or irregular objects correctly.  Accuracy of measurement when using a caliper is highly dependent on the skill of the operator. Regardless of the type, a caliper’s jaws must be forced into contact with the part being measured. As both the part and caliper are always to some extent elastic, the amount of force used affects the indication. A consistent, firm touch is correct.  Too much force results in an underindication as part and tool distort; too little force gives insufficient contact and an overindication.  This is a greater problem with a caliper incorporating a wheel, which lends mechanical advantage. This is especially the case with digital calipers, calipers out of adjustment, or calipers with a poor quality beam.

Simple calipers are uncalibrated; the measurement taken must be compared against a scale. Whether the scale is part of the caliper or not, all analog calipers—verniers and dials—require good eyesight in order to achieve the highest precision. Digital calipers have the greatest advantage in this area.

Calibrated calipers may be mishandled, leading to loss of zero. When a caliper’s jaws are fully closed, it should of course indicate zero. If it does not, it must be recalibrated or repaired. It might seem that a vernier caliper cannot get out of calibration but a drop or knock can be enough. Digital calipers have zero set buttons.

Vernier, dial and digital calipers can be used with accessories that extend their usefulness. Examples are a base that extends their usefulness as a depth gauge and a jaw attachment that allows measuring the center distance between holes. Since the 1970s a clever modification of the moveable jaw on the back side of any caliper allows for step or depth measurements in addition to external caliper measurements, in similar fashion to a universal micrometer.

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