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A student measured the diameter of a small steel ball using a screw gauge of least count 0.001cm. The main scale reading is 5mm and zero of circular scale division coincides with 25divisions above the reference level. If the screw gauge has a zero error of 0.004cm, the correct diameter of the ball is?
(A) 0.053cm
(B) 0.521cm
(C) 0.529cm
(D) 0.525cm

Answer
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Hint
The screw gauge is defined to be an instrument that is used for measuring exactly the diameter of a thin wire or the width of a sheet of a metal. It comprises a U-shaped mount which is fixed with a screwed pin which is fixed to a thimble. The least count of a screw gauge is 0.01mm. Based on this concept we have to find the answer to this question.

Complete step by step answer
The Least count of the screw gauge is =0.001cm
The main scale reading of the screw gauge is R=5mm and 25divisions above the reference level.
Zero error in the screw gauge =0.004cm
Observed reading of the screw gauge=0.5+25×0.001
=0.5+0.025
=0.525cm
Actual reading =0.525+0.004
=0.529cm
Therefore, the correct answer is Option (C).

Note
We should know the smallest value that can be measured by the measuring instrument is known as the least count of that instrument. We should also know that the least count error is defined as the error that is associated with the revolution of the instrument.
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