Spherometer - Double Disc

It consists of a small metal frame, supported by three legs fixed at the corners of an equilateral triangle. A screw of fine pitch passes through the centre of the metal frame. The screw forms the fourth 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 centre (10-0-10mm). A circular scale is fixed to the screw head. It carries a circular scale divided into 100 equal parts.