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What Is a Spherometer?

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  • Written By: Michelle Baugh
  • Edited By: C. Wilborn
  • Last Modified Date: 20 November 2014
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A spherometer is a device used in mechanical engineering to obtain the precise measurement of the radius of a circle. It is a unique measuring tool in that it can accurately measure curved surfaces. The tool was originally designed to give opticians the ability to measure the curvature of glass lenses without having to rely on expensive interferometry equipment.

Most spherometers are constructed of a three-legged base that takes the shape of a equilateral triangle. All of the legs taper off into fine points at the bottom. In the middle of the base is a screw with a fine point that is attached to a nut and a measurement dial. There is usually a vertical scale attached to the side of the spherometer, which is used to indicate how many whole turns of the nut were made, as well as to provide an index for reading the fine measurement dial attached to the screw.

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To get a reading from a spherometer, the instrument has to be completely leveled. Then it can be placed on the object to be tested. All four of the fine points must be exerting an equal amount of pressure before the measurement can be read. The difference between the measurements shows the thickness of that area, which is cut off by a plane that is passing through the three equidistant feet. Using r to denote the overall distance and s for the distance between the feet, the radius can be determined by using the formula R=r2+s2/2s.

There are also digital spherometers, which usually come in two forms. The first form allows users to obtain a measurement directly on the device — instead of a measurement dial, the screw will have a plate with a digital face on it. The second form involves a spherometer with no face reading, connecting instead to a separate device that prints out a digital reading. A digital spherometer device may be easier for users to read, but there is some concern that it comes at a cost to accuracy. A digital unit may not be able to record the roughness on a surface as well as a standard dial spherometer can.

Although spherometers are still commonly used by opticians, they can also be found in other fields, usually as quality-control devices. For example, a spherometer might be useful for testing drill pipes before shipping them out to companies to make sure they won't fracture while in use. It can also be used by anyone who needs to measure the thickness of thin or flat plates.

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