Question

how to use a spherometer?

Answer 1

To do so, the instrument is placed on a perfectly level plane surface and the screw turned until the point just touches; the exact instant when it does so is defined by a sudden diminution of resistance succeeded by a considerable increase. The divided head and scale are read; the screw is raised; the thin plate slipped under it; and the process is repeated. The difference between the two readings gives the required thickness.

Similarly, the instrument can measure the depression in an otherwise flat plate. The method would be as for measuring the thickness of a plate, except that the micrometer portion is placed over the depression and the measurement is taken below the surface instead of above.A related device is the Cylindrometer (also known as Cylindro-Spherometer and Sphero-Cylindrometer), which can additionally measure the radius of curvature of a right-circular cylinder