This is used for the accurate measurement of the radius of curvature of spherical surfaces, such as lenses.
It also finds application in measuring plate thicknesses and depressions.
The device consists of a tripod (three- pronged aluminum table) with three pointed steel tips which form an equilateral triangle.
A micrometer screw with a measuring tip is recessed in the middle.
Attached to the micrometer screw is an aluminium disc with circular divisions from 0 to 100.
Also there is a vertical scale with millimeter divisions from -10 to 10 mm.
The vertical scale can be provided with alternate millimeter divisions, that is in different ranges, from -10 to15mm or from -7 to 7mm.
Henceforth the measuring ranges include:
0-20mm and -10-10mm
0-25mm and -10-15mm
0-14mm and -7-7mm.
The screw pitch =1mm.
Measuring accuracy (least count) = 0.01mm.