A slide caliper is simply a rule with a fixed abutment, like a hook rule , plus a sliding head . The head is arranged with a knife edge that crosses the rule exactly in line with the movable jaw, allowing direct reading of the dimension against the rule markings. Usually there is also a means to lock the head, to avoid disturbing the reading when the tool is withdraw from the work. Such tools still exist, just as described, but while they add convenience ( and probably improve repeatability because of the â€œcursorâ€?function of the sliding jaw), their precision is limited by the resolution of the ruleâ€”never finer than 0.010 inch.
A very early modification to the slide caliper that greatly improved its precision was the introduction of the Vernier scale, invented by a French, mathematician Pierre Vernier(1580-1637). Although vernier calipers are now losing popularity to dial and digital calipers, many machinists and mechanics still have a fondness for them . This is partly because they are somewhat more compact (dial on dial calipers occasionally gets in the way), and partly because they may have concerns about the accuracy of dial calipers over the long haul (wear of the moving parts over time can potentially introduce some â€œbacklashâ€?. Partly, too, they may simply enjoy exercising an ancient skill that is gradually becoming extinctâ€”see the sidebar â€œSplitting the Difference.â€?br /> On inch-measuring dial calipers, the main scale is usually divided into 1/10-inch (0.1) units, while each dial marking represents 0.001 inch; one full revolution of the dial corresponds to one division on the main scale. Beware, however, that you should carefully scope any unfamiliar dial caliper the first time pick it up: There are some models that use 0.2-inch units on the main scale. Thus, while a full revolution of the dial still corresponds to one main scale division, those revolution are twice as big. Likewise each marking on the dial still represents 0.001 inch, but there are twice as many marks.