What do you mean by backlash error of spherometer?

The backlash error of a spherometer refers to the discrepancy in measurements due to the presence of mechanical play or slack in the instrument's components. This error occurs when the thimble or spindle of the spherometer moves slightly before the actual contact is made with the surface being measured, leading to inaccuracies in the readings. To minimize backlash error, it is important to ensure that the spherometer is properly calibrated and that any slack in its components is eliminated through careful adjustment and maintenance.