Physics · Units And Measurements · NEET
Always SUBTRACT the zero error, keeping its sign: Correct reading = Observed reading - Zero error. If the zero error is negative, subtracting a negative number means you actually add its size. Example: if zero error = -0.004 cm, then Correct = Observed - (-0.004) = Observed + 0.004 cm. So the correct value comes out bigger.
Close the jaws with nothing in between. If the zero of the moving scale is AHEAD of the main-scale zero (reads a small positive value like +0.03 cm), that is a POSITIVE zero error - the instrument reads too high, so you subtract it. If the zero of the moving scale sits BEHIND the main-scale zero (below the reference line, appears to read a negative value), that is a NEGATIVE zero error - the instrument reads too low.
They are opposite in sign. Zero correction = -(Zero error). So Correct reading = Observed reading + Zero correction. Many NEET questions give you the zero error directly; just plug it into Correct = Observed - Zero error and the sign takes care of itself. Do not apply both a correction and a subtraction - use only one form.
On an optical bench you measure distances using the tips of pins or the position marks of lens/mirror holders. The tip of a pin usually does NOT sit exactly at the scale mark of its holder. Index correction is the fixed length you add or subtract to fix this gap so the measured object distance and image distance are true distances from the lens. It is zero error for the whole bench setup, not for one instrument.
Close the studs with nothing between them and read the circular scale against the reference line. If the circular scale zero is above the reference line by 4 divisions and least count is 0.001 cm, the zero of the circular scale reads below the line, so zero error = -4 x 0.001 = -0.004 cm (negative). If it is below the line by n divisions, zero error = +n x least count (positive).
Index correction is specifically used on the optical bench, for example when measuring the focal length of a lens or mirror using pins on an optical bench. A meter bridge uses end corrections (a similar idea, different name), a simple pendulum and a resonance tube do not need index correction. NEET 2026 asked exactly this: focal length of lenses using optical bench is the answer.
A student measured the diameter of a small steel ball using a screw gauge of least count 0.001 cm. The main scale reading is 5 mm and zero of circular scale division coincides with 25 divisions above the reference level. If the screw gauge has a zero error of -0.004 cm, the correct diameter of the ball is:
The diameter of a spherical object is measured with a vernier caliper whose 10 vernier scale divisions (VSD) equal 9 main scale divisions (MSD). The least MSD is 0.1 cm and the zero of the vernier scale is at x = 0.1 cm when the jaws are closed. If the main scale reading for the diameter is M = 5 cm and the coinciding vernier division is 8, the measured diameter after zero-error correction is:
Which of the following measurements requires 'index correction'?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Zero error is a systematic (instrumental) error. It shifts every reading by the same fixed amount and in the same direction, which is why it can be fully removed by simple correction. Random errors cannot be removed this way.
Correct reading = Observed reading - Zero error, where the zero error is written with its own sign (positive or negative). Equivalently, Correct reading = Observed reading + Zero correction, where Zero correction = -(Zero error).
Yes. Even a well-made vernier caliper or screw gauge can develop a small zero error from wear or manufacturing. That is why you always check the closed-jaw reading first and note the zero error before measuring.
No. Zero error only shifts the reading up or down by a fixed amount. The least count depends only on the scale design (least count = 1 MSD - 1 VSD for vernier, pitch / number of circular divisions for screw gauge).
Because the rule Correct = Observed - Zero error applies with sign. When zero error is negative, the instrument was reading too low, so subtracting a negative value adds it back, correctly increasing the reading.
