The cartwright's wheel consists of a circular disk, made of wood or iron, mounted on a wooden forked handle. It was used to measure the circumference of the wheel, in particular to define the length of the metal bar needed to build the outermost part of the wheel (the rim).
To achieve this, from an initial measurement point marked on the wooden wheel, the measuring wheel was rolled along the circumference until it returned to the starting point. By counting the number of revolutions, the exact circumference was calculated and then transferred to the metal bar used to make the rim.