In a screw gauge, the zero of the circular scale lies divisions above the horizontal pitch line when their metallic studs are brought in contact. Using this instrument thickness of a sheet is measured. If pitch scale reading is and the circular scale reading is then the correct thickness of the sheet is _____ . [Assume least count is ]
Correct answer:B
Standard Method
Given: Least count is , pitch scale reading is , circular scale reading is , and the zero of the circular scale lies divisions above the horizontal pitch line when the studs are in contact.
Find: The correct thickness of the sheet.
When the zero of the circular scale is above the reference line, the zero error is negative, so its magnitude is added to the observed reading.
The zero error is
The observed reading is
Now apply zero correction:
Therefore, the correct thickness of the sheet is . The correct option is B.
Students often subtract because they see a zero error and assume every correction is subtraction. Here the zero is above the line, so the zero error is negative and its magnitude must be added to the measured value.
A common mistake is to use only the circular scale reading and forget the pitch scale reading. The measured value is , so must be included.
Some students treat as directly without multiplying by the least count explicitly. Always convert circular scale divisions using before adding to the pitch scale reading.
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