Bar gauges are used while adjusting several classes of smiths' work to a stated length or width; for this purpose, two dots are put into two places of a rod or bar, and the work is afterwards lengthened or shortened until the distance between the centres of the two dots is the length required. This desired length is the distance between the two centres of the two pointed extremities of the gauge represented by Fig. 238. Such a gauge is quickly made, in cases of emergency, by bending any piece of wire or small rod which is near ; but the suitable material is a broad thin bar, that the gauge may be easily carried about, and not liable to bend in the mid portion, which renders it an uncertain measure.

A bar gauge which is made by bending the iron or steel edgeways is good enough for many purposes; and if the ends are thinned to a taper form, the gauge is much lightened without impairing its efficiency. A gauge of this shape is tapered, and the ends bent, also the small points made and smoothly filed, previous to adjusting the gauge points to that distance from each other which is required. This adjustment is effected by means of a hammer, and one of the divided bines on the surface of a table similar to that described in the section on Tables. After applying the two gauge points to the stated length on the measure, the gauge points are separated from each other, if too near; and put nearer to each other, if too far apart. When discovered to be too far apart, the curved junctions of the arms with the remainder of the gauge are heated, and the arms are driven towards each other with a few blows; but if only a small amount of alteration is needed, it is given while cold, by placing the gauge on some hollow place, with the inner edge of the gauge upwards, and giving a few light blows with a hammer. When the gauge points are found to be too near each other, they are separated by hammering the outside edge of the gauge, instead of the inner edge; but if much lengthening is required, the gauge is heated in the mid part and stretched with hammering.