Model 757W1 measures 50 to 4000 RPM shaft speed on any open-ended shaft instantly. Reads directly in RPM. Accurate within plus or minus 2%. 2-1/2" diameter dial. Scale is divided in 50 and 100 RPM steps. Large numerals every 500 RPM. Has magnetic damper for steady pointer indication, with cup shaped tip for flat-end shafts, cone-shaped for shafts with center holes. Magnetic mechanism is enclosed in dust and waterproof steel case. 2-3/4" diameter and 4" long overall. Weight is 14 oz.