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Question
What is meant by zero error of vernier callipers ? How is it determined? Draw neat diagrams to explain it. How is it taken in account to get the correct measurement?
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Solution
Due to mechanical errors, sometimes the zero mark of the vernier scale does not coincide with the zero mark of the main scale, the vernier callipers is said to have zero error.
It is determined by measuring the distance between the zero mark of the main scale and the zero mark of the vernier scale.
The zero error is of two kinds
(i) Positive zero error
(ii) Negative zero error
(i) Positive zero error: On bringing the two jaws together, if the zero mark of the vernier scale is on the right of the zero mark of the main scale, the error is said to be positive.
To find this error, we note the division of the vernier scale, which coincides with any division of the main scale. The number of this vernier division when multiplied by the least count of the vernier callipers, gives the zero error.
For example, for the scales shown, the least count is 0.01 cm and the 6th division of the vernier scale coincides with a main scale division.
Zero error = +6 × L.C. = +6 × 0.01 cm
= +0.06 cm
(ii) Negative zero error: On bringing the two jaws together, if the zero mark of the vernier scale is on the left of the zero mark of the main scale, then the error is said to be negative.
To find this error, we note the division of the vernier scale coinciding with any division of the main scale. The number of this vernier division is subtracted from the total number of divisions on the vernier scale and then the difference is multiplied by the least count.
For example, for the scales shown, the least count is 0.01 cm and the sixth division of the vernier scale coincides with a certain division of the main scale. The total number of divisions on vernier callipers is 10.
Zero error = - (10 - 6) × L.C.
= - 4 × 0.01 cm = - 0.04 cm
Correction:
To get correct measurement with vernier callipers having a zero error, the zero error with its proper sign is always subtracted from the observed reading.
Correct reading = Observed reading - zero error (with sign)
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