My screw gauge shows a positive reading when fully closed. What correction should I apply?
That nonzero closed reading is a positive zero error.
That nonzero closed reading is a positive zero error. Write it algebraically and subtract it from every observed reading: corrected = observed - positive error. If the zero error were negative, subtracting a negative value would add its magnitude. State the least count and units, and avoid memorising 'always add' or 'always subtract' without the sign.
Key point
Correction is always the negative of the algebraic zero error.
Common mistake
adding a positive zero error to the observation.
Memory tip
observe minus error.
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