Originally Posted by meanderthal

I have the one that 2manybikes cited. It's brand name is ALVIN, and is quite accurate if used carefully. Make sure, when you start a trace, that you have first rolled the wheel backwards (to the underside of the zero mark) and then back to zero, as there's a bit of play in the wheel-to-pointer linkage. If you don't, the pointer may not begin moving until you've moved a bit past the start point, and your first readout will be short by that amount.

Lew