Hi, i came across this applet which i heard from lookang.
In general, it's a great applet which closely simulates a real vernier caliber, with the lock and the zero error in place.
However, i note that when the black box is fully dragged down, it overlaps the vernier caliber. Perhaps a correction can be done such that the maximum displacement is not as much when the black bar on the left is fully dragged down. And also i think that a hint & answer for zero error can be done so as to increase awareness of its existence and how it affects the actual reading.
Nevertheless i find this is a great applet! Thanks for the effort!