Product Description The Starrett 79B-4 “Yankee” spring-type outside caliper opens to 4” (100 mm) with sturdy construction and smooth action. The threads of the spring nut firmly engage the screw with pressure from the leg. When pressure is withdrawn, the nut automatically releases itself and slides freely over the screw, saving time in opening and closing. It is made of flat stock, high-grade steel, for easily reproducible and accurate transfer of measurements of the external size of an object. This caliper has flat, inward curving legs for outside measurements and comes with a spring nut. Bow spring is flexible yet strong, for reliability. The durable legs are made of flat stock, high-grade steel for long life. The centrally located adjustment rod ensures smooth action when opening and closing the caliper. The hardened fulcrum stud is strong to help prevent give or play in the measurement, which would reduce accuracy. Calipers measure the distance between two opposing sides of an object. They make inside, outside, depth, or step measurements, according to their type. Calipers are commonly used in architecture, metalworking, mechanical engineering, machining, manufacturing, carpentry, and medicine. The simplest calipers have two legs to mark the two points and require a ruler to take the measurement. More complex calipers use two sets of jaws instead of legs and have up to two graduated scales. Vernier, dial, and digital calipers give direct and accurate readings and are functionally identical, having a calibrated scale with a fixed jaw, and another jaw with a movable pointer that slides along the scale. The vernier caliper has a scale sliding parallel to the main scale for an additional, fractional reading to improve measurement precision. The dial caliper has a circular dial with a pointer on a toothed gear rack replacing the second vernier scale. As with the vernier, this second measurement is added to the reading from the main scale to obtain the result. The dial caliper is used also for measuring size differential between two objects. The digital caliper takes the same sort of differential measurements as the dial caliper by zeroing the display at any point along the slide, with an LCD (replacing the dial) that displays a single, easily read value in both English and metric units. Some digital calipers can hold data readings between measurements and send them to data collection devices. The L. S. Starrett Company was founded in 1880, originally manufacturing bench vises, squares, and other tools. They acquired other companies throughout the years, expanding their services into making precision measurement instruments and tools, such as calipers, micrometers, saw blades, and gauges. Starrett is headquartered in Athol, Massachusetts and has manufacturing facilities in Brazil, the United Kingdom, China, and other locations. From the Manufacturer Starrett "Yankee" Calipers are made from a high-grade steel and well-finished. The legs are made of flat stock and are very durable. The fulcrum stud is hardened and has a smooth bearing surface. The bow string, although flexible, is exceedingly strong. The Starrett quick-adjusting automatic-closing spring nut is designed for making fast, positive adjustments. The threads of the nut firmly engage the screw at the slightest pressure from the leg. When the pressure is withdrawn, the nut automatically releases itself, sliding freely over the screw.