The spherometer consists of a central steel support with an inset screw, which forms a sppike of variable height (1mm scale) and a rotaing disk fixed vertically to the screw, with graduations to measure the height of the disk on a 1/100 scale. Adjusted to the support are three spike-shaped legs at equal angles of 120degrees.
Used to measure the radius of curvature of an extended object, for example a lens, to reasonable precision. The vertical scale is used as a height measurement for the screw (with the disk as a marker), and the graduations on the disk as a fine-resolution device for the same measurement, to fractions of a revolution. Adjustment of the screw, to place all four legs on the surface, allows calculation of the radius of curvature from the height measurement and the dimensions of the device.