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.
4)micrometer screw gauge
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.