How to use a spherometer?

how to use a spherometer?
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A spherometer is an instrument to measure very small lengths with precision. As its name reflects, it is used to measure the radii of curvature of spherical surfaces.
It is based on the principle of screw.

In general the spherometer consists of:

  1. A base circle of three outer legs, a ring having a known radius of the base circle.
  2. A central leg, which can be raised or lowered.
  3. A reading device for measuring the distance the central leg is moved.
    Present spherometers have a vertical scale marked off in units of 0.5 mm.
    One complete turn of the dial also corresponds to 0.5 mm and each small graduation on this dial represents 0.005 mm.

Procedure to use a spherometer:

  1. The instrument is first placed on a perfectly plane surface and the middle foot screwed down until it touches the surface. As soon as this is the case, the instrument begins to turn round on the middle foot as a centre.
  2. The spherometer is then carefully removed from the glass, and the reading of the micrometer screw is taken. If the instrument is perfect, the reading should be 0-0. However, there is always a slight error in instrument (just like zero error in vernier calliper). It could be positive or negative.
  3. Now take the instrument off the plane and draw the middle foot back some way. We will suppose we are going to measure the radius of a sphere from the convex side.
  4. Read the scale and screw-head. Suppose the reading is 2.0 and 0.225, then the total reading is 2.225
  5. This distance should be subtracted from the zero error (2.225 - zero error). Or it should be added if the reading was below the zero line.
  6. Measure the length between the two legs by placing it on card paper and taking the length with a metre scale.
  7. Calculate the radius of curvature as
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