# Measuring small changes in length using the micrometer screw gauge

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.

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 $$C$$.