Spherometers are instruments designed to measure the radius of curvature of spherical surfaces.~(1,2) They are particularly useful in situations where only a portion of the spherical surface is available, for example, for measuring the radii of curvature of spherical lenses.~3 A spherometer can be easily modified so that it can also be used to measure the radius of curvature of a right circular cylinder. The resulting device is called a cylindrometer (also known as the cylindrospherometer). The idea is quite old~4 but is seldom mentioned in introductory laboratory courses. This paper describes the device and its operation.
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