Calipers are instruments for comparing and checking sizes. They are made of wood, plastic and metal and each unit has two arms which join at one end and are curved at the other end. Both arms can be moved 180º, allowing the ends to be pointed inwards or outwards. At the joint they can be tightened so the size is exact when measuring and comparing sizes.
Drawings from left to right
a) Caliper to measure the outer edge of a flange.
b) Caliper to measure the inner edge of a flange.
c) Caliper joined; an accurate way to size lids for pots so they fit exactly. When you measure the size of one, the other moves in the opposite direction so you have measurements for the inside of the neck and the outside of the lid.