Common length-measuring instruments

Standard length-measuring instruments are, by definition, instruments in common use, requiring neither lengthy training nor a high level of knowledge, and therefore using simple measurement principles. Today's so-called standard instruments are generally those that were already in use in the course of the last century, or even earlier in the case of calipers and micrometers, whose first instruments date back to the end of the 18th century. The principle of the vernier consists in placing an engraved ruler, close to the rule, most often on a bevelled part above it. The most common vernier is the 1/50 mm vernier: the ruler is engraved every millimeter and the vernier consists of 50 strokes by 49 mm; the 1/20 mm vernier has 20 strokes by 19 mm, and the 1/10 mm vernier (which is practically no longer used today) has 10 strokes by 9 mm.

The micrometer, often still called a palmer after its inventor J.L. Palmer in 1848, consists of a precision screw, read by a graduated drum for 1/100 mm instruments and sometimes by a vernier for 1/1,000 mm instruments.

Comparators have enabled the development of a whole new metrology. It's true that all measurement is a comparison, but habits are such that the term "comparison" is reserved for measuring methods in which the part to be measured is "compared" with a standard of the same nature, i.e. the same surface shape and dimension close to the part's size. This last statement remains valid, in principle, for short-stroke comparators, for which the principle of measurement (known as the Borda method) consists of setting the comparator to "zero" on the standard and then measuring the part to determine the deviation from the standard. For dial gauges with a measuring travel of several millimetres - for example, the most common dial gauge has a travel of 10 mm - it does not measure a small deviation, and is therefore a displacement gauge. This is why the name dial gauge is sometimes controversial for these instruments.

The development of electronics has given rise to new instruments such as electronic displacement sensors and fringe-counting interferometers. But conventional instruments have also evolved with these technologies, in particular with the generation of so-called digital display instruments. Today, all common instruments have a digital display. The principles used for these instruments are of two types. The most commonly used devices are of the capacitive type (see diagram). The capacitive sensor is integral with the ruler for slide-type instruments (calipers and slide-type depth gauges), with the reader with its electronics integrated into the slide; for certain digital display comparators, the same device (sensor and linear reader) is used for screw-type instruments, external micrometers, screw-type...