2, the relative error is equal to the difference between the measured value and the real value is greater than the real value.

The length of the steel wire is very long, so the absolute error is great even if the meter ruler with lower accuracy is used, but the relative error obtained after comparing with its real length is not too great; The diameter of steel wire is very small, so the measuring instrument needs high precision to get the ideal relative error.

3. Parallax is a phenomenon that it is difficult to carry out accurate experimental measurement with the eye shaking (the observation position changes slightly) and the scale moves relative to the measured object during the adjustment of optical experiments.

Try to keep your eyes level, take the average after many measurements, and adjust the reticle of the eyepiece (I am a junior, and I can't remember some professional terms clearly. Take a good look at the experiment book. The teacher should also mention the thinking problems in the experiment)