Calipers are used to measure openings and galleries in lidded forms, such as jars. They can also be used to take other measurements, as well. For example, you may want to throw a cup with a saucer that has a middle depression that fits the base of the cup exactly.
In these cases, you need one thrown form to fit as closely as possible to another. Calipers are used to measure the diameter of both the opening (or depression) and the part that will fit into the opening.
In this example, the opening in a pot for a lid is being measured. (Here, the gallery is part of the pot. Depending upon the type of lid you are throwing, the gallery may be on the lid or on the pot.) Since the lid will fit down inside this area, the measurement needed here is the diameter of the opening.
- Loosen the calipers' wingnut so that you may easily move the arms, but not so much as to free them from friction completely.
- Slide the arms of the calipers so that they point outward. Move them so they are as close as possible to the edges of the opening, without digging into the clay.
- Be certain that the calipers are going across the exact middle of the opening, not cutting across a shorter distance.
- Tighten the wingnut so the arms cannot shift. Double check the measurement.