Using Vernier calipers. This measuring device is being used to measure the diameter of a metal ball bearing. The external jaws are being used. The other jaws (above ball bearing) are the internal jaws, used to measure internal distances such as the diameter of a hole. Vernier calipers are calipers (a type of measuring device) that have a Vernier scale to allow accurate measurements. The jaws are moved using the wheel (under thumb), and the value is read from the scale in the oval area at centre right. The scale here is given in both centimetres (lower scale) and inches (upper scale). The scale marked on the oval window (the Vernier scale) allows measurements to be made with increased accuracy, here to a 10th of a millimetre or to 128th of an inch. The scale shows that the ball bearing is 1.59cm (0.625 inches) wide.