calipers

Calipers are a measuring instrument used to determine the distance between two opposite sides of an object. They come in several forms, most commonly vernier calipers, dial calipers, and digital calipers. All variants have external caliper jaws for outer measurements, internal jaws for inner dimensions, and a depth rod for measuring depths.

The concept dates from ancient times, but the vernier caliper—named after the French mathematician Pierre Vernier—was introduced in 1631 to obtain finer readings. Modern calipers are typically made from stainless steel or other hardened alloys, with some inexpensive models in plastic. They can be operated in metric or imperial units, and many offer switching between scales.

Range and accuracy: Common stock calipers measure up to about 150 millimeters (6 inches). Reading methods vary by type: vernier calipers use a secondary scale to interpolate, dial calipers use a rotating dial, and digital calipers show a numeric reading on a display. Typical precision is around 0.02–0.05 millimeters (0.001–0.002 inches), with higher-grade instruments available.

Use and maintenance: To measure, place the object between the jaws and slide until snug, then read the measurement. Zero the instrument before use, and keep it clean and dry. Periodic calibration and proper storage extend accuracy.