How do you prevent parallax error when reading a scale?

Keep your eye directly perpendicular to the scale and use a mirror or pointer to make your sight line true to the measurement.

Parallax error happens when your eye is not lined up with the mark you are reading, so the pointer or needle looks shifted against the scale. Imagine looking at a ruler from the side and the zero mark seems moved; that is parallax. To avoid it, move your head until you see the mark and the pointer in a straight line, as if both are on the same flat surface. Using a mirror behind the pointer helps because you adjust your view until the real pointer and its mirror image line up, which means you are looking straight on.

Students often read a scale from an angle because it feels more comfortable, which gives the wrong value, so make a habit of moving your eye rather than tilting the instrument. Another common mistake is assuming a small angle does not matter, but even tiny misalignment can change precise measurements, so check alignment every time. Sometimes people rely on memory for where they last read instead of rechecking sighting, so always re-sight for each measurement.

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Preventing Parallax Error When Reading Scales

1. Line up eye with mark
2. Move head until pointer aligns
3. Visualize scale and pointer flat
4. Use a mirror behind pointer
5. Adjust view until images match

Common Parallax Mistakes and Fixes

1. Reading scale from an angle
2. Assuming small misalignment is negligible
3. Relying on memory instead of rechecking

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