Free
https://itunes.apple.com/us/app/vernier-calipers-simulator/id1163657388?ls=1&mt=8
Pro
https://itunes.apple.com/us/app/vernier-calipers-simulator-pro/id1244019319?ls=1&mt=8

About
An open source physics at Singapore simulation based on codes written by Fu-Kwun Hwang and Loo Kang WEE.

Introduction
Vernier calipers can measure external dimensions using the pictured lower jaws.
The Vernier calipers model has
an object (Blue) for the internal jaws to measure width of an object with slider to control width of the object and simple drag action to control position of object.
a slider for zero error slider
checkbox for hint to scaffold the learners attention and point of focus
checkbox for answer to show the meaning of reading on the main scale and the vernier scale with zero error calculations if any.
drop down menu of the various common vernier scales for sense making and additional testing out by learners their ideas of how vernier works.
fine control buttons for learners to manipulate the model with single incremental precision
slider control for fast changes in the vernier measurement
reset button to bring simulation back to original (default) setting.

Acknowledgement
My sincere gratitude for the tireless contributions of Francisco Esquembre, Fu-Kwun Hwang, Wolfgang Christian, Félix Jesús Garcia Clemente, Anne Cox, Andrew Duffy, Todd Timberlake and many more in the Open Source Physics community. I have designed much of the above based on their ideas and insights, and I thank the OSP community for which Singapore was honored with 2015-6 UNESCO King Hamad Bin Isa Al-Khalifa Prize for the Use of ICTs in Education.