Powerful rare earth magnets have superior holding power on ferrous metal surfaces . Oversized solid aluminum frame with machined working edges assure maximum accuracy. Oversized 1/2" diameter vials are easy read; oversized top-read windows for easy viewing. "Quik-Change" replaceable cover plates and vials for in the field service. "SurroundView" vial system for enhanced readability; oversized Anti-Slip/Anti-Shock end caps protect frame and rubberized cushioned hand grips.