Friction thimbles offer a smooth measuring operation and resist shock. They do not offer audible feedback, but are designed to be closer to the operators hand.
Ratchet stop thimbles offer audible feedback and constant measuring pressure on the spindle while measuring. They cause micro-shocks and requires two hands to make a measurement.
Parallelism: [2 + L/100][.00008 + .00004(L/4)] where L = Maximum measuring length