The vernier caliper is an extension of the slide caliper. By adding vernier scales, we can increase their precision by an order of magnitude. The vernier scales by themselves do not improve the accuracy of the instrument: if the primary scale of a vernier is inaccurate, we could make very fine measurements of the wrong dimension. Primary scales, however, can be produced to much finer precision and accuracy than the eye can resolve.
It is not true that we can achieve greater perfection or â€œexpand the scaleâ€?only by the vernier scale. Nowadays, we have begun to use a dial, for the least count readout and electronic digital readouts. These calipers are appropriately called dial calipers.
Dial mechanism will malfunction more often than the simple vernier scale , and the accuracy of the reading mechanism of the dial caliper depends on how far it travels. Normally, dial calipers are accurate to 20 Ð§m per 150 mm(0.001in. per 6 in.) of travel. In contrast, a vernier reading has the same level of accuracy any place along the scale, but you must always, at the same time, consider the accuracy of the main scale.
Similar to vernier calipers, dial calipers have a stationary bar and a movable slide assembly. The bar is graduated in increments of 0.1 in. (2mm) and is available in sizes ranging from 4-12 in. (100-300mm). The vernier plate is replaced by a caliper dial graduated in increments of 0.001 in. (0.02mm). A pinion gear actuates the dial hand as it moves along a rack located in the stationary bar. Because the dial caliper is direct reading , there is no need to determine the coincident line on a vernier scale. This feature facilitates the reading of these instruments. Dial heads are also incorporated on caliper height gages and depth gages.