Conventional calipers are used to make a continuous recording of the gauge and rugosity of a wellbore. Conventional calipers are available in one, two, three, and four arm versions (a few exotics have been made with six or more arms). As a generalization, one and two arm calipers tend to measure the maximum dimension of a non-round hole, while three arm calipers tend to measure something closer to the minimum dimension.
Since the 1940's, calipers have been an important part of open hole logging. A caliper may be used to center or eccenter a tool string (sometimes also used in the same fashion with cased hole tools); a caliper-like arm may carry sensors or a radioactive sealed source. A caliper is of great importance in the determination of the validity of open hole logs. Modern technology can use the caliper to correct certain open hole curves. The savvy petrophysicist may even be able to draw conclusions about where net productive intervals are located since mud filter cake buildup occurs across permeable zones resulting in under gauge caliper readings (depending on arm tip type).
But calipers are the most fun to run in shot holes or open hole completions. In this application, the caliper can be regarded as a cased hole or production logging tool. It is interesting to compare a shot hole caliper log to lithology logs since the softer and harder zones within the productive interval are often easily recognized (the softer material tends to cave more after the shot, leaving a bigger hole).
The arm movement of a well logging caliper must be translated into usable data. Early caliper designs were either purely mechanical or used resistive elements. There have been dozens of schemes over the years, including elaborate mechanical contrivances, hydraulic pressure transducers, magnetically altered coils, mechanical movement of internal vacuum tube elements, magnetic flux gates, magnorestrictive elements, and on and on. While there are still some exotic caliper transducer schemes floating around, most modern calipers utilize a variable resistor a/k/a potentiometer or "pot" somehow connected to the movable arms in both dc and pulse tools. Comprobe offers an interesting magnetically coupled bow spring caliper that can double as a centralizer, as an example of one of the more unusual calipers presently manufactured.