The surface to be tested runs conveniently underneath this rolling contact point. The roller allows for continuous readings to be taken. Used with flat metals, foils, sheets of fabric, etc. A lock nut (included) keeps the roller aligned.
Available in non damaging Teflon®, hardened steel or stainless steel. The wheel can have a radius or it can be flat. You will probably want to use the radius wheel on a flat surface and the flat wheel (without radius) when measuring something like wire or edges.
The stainless roller without radius is not as hard as hardened steel (your other option) and will wear faster under abrasive circumstances, but it will not corrode if used under wet or damp conditions.
Stainless steel rollers will be a natural steel color whereas hardened steel will have some black anodizing on the wheel.