It
consists of a small metal frame, supported by three legs fixed at the corners
of and equilateral triangle. A screw of fine pitch passes through the center of
the metal frame, the screw forms the forth leg. The main scale (or pitch scale
is marked on a metal strip fixed at right angles to the frame, this scale is
marked in millimeters with zero mark at the center (10-0-10 mm). A circular
scale is fixed to the screw head.
