The Magnet probes were designed to be used on ferrous material. It has four magnets to insure plenty of holding power to the part.
There are 3 Magnet probes, flat,concave and convex. The flat is for inside surfaces that are flat. The convex is for inside of round pipe and the concave is for the outside of a cylinder.
The Magnet Probe is .44" x .475" x 2.7".