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Which of the following method/instrument cannot be used to measure small changes of the order of a millimeter occurring in a length of about 50cm.

1)spherometer

2)travelling microscope

3)meter ruler

4)micrometer screw gauge

5)lever method

This is a question in a college examination I sat for recently. The given answer is (4). I don't understand how a spherometer could be used either. The rest seem to be possible. Please explain me whether I have understood it wrong or whether both 1 and 4 are possible answers.

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One is looking for changes in length and so one does need to measure the full length.

The arrangement might be as follows with one end of a rod $A$ fixed to the housing of a screw $B$ via a rigid base.

As end $D$ of the rod moves the change in length of the rod can be monitored by using screw $C$.

enter image description here

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  • $\begingroup$ Yeah I got your point.Every other instrument can be used to measure the CHANGE except for the micrometer screw qauge since it has a maximum measurement because of its u-shaped frame. $\endgroup$
    – AfiJaabb
    Commented Jul 22, 2019 at 12:00
  • $\begingroup$ @AfiJaabb That is not what I meant. It is the meter rule which is the odd one out as it cannot measure mm changes accurately. $\endgroup$
    – Farcher
    Commented Jul 22, 2019 at 12:03
  • $\begingroup$ The least count of the meter rule is 1mm and the whole measurement is 50cm.This means the fractional error of the measurement is very small.So why would you call it inaccurate? $\endgroup$
    – AfiJaabb
    Commented Jul 22, 2019 at 12:07
  • $\begingroup$ @AfiJaabb It is because you are asked about a difference of the order of mm not the actual length. $\endgroup$
    – Farcher
    Commented Jul 22, 2019 at 12:09

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