A light radius gauge, denoted by Fig. 242, is used for measuring distances between any two places on a measure, and also for adjusting a forging to the desired length or width ; a radius gauge is therefore a substitute for the bar gauge shown by Fig. 238 ; a radius gauge is also a superseder of a simple bar gauge, because the points or scribers are capable of adjustment at any distance between each other within the limits of the instrument. Another important use for such a gauge is that of exscribing arcs or circumferences of various radii; the required length of radius being obtained by shifting the scriber-holders to the proper places on the bar, and fixing the scriber-holders with the fixing screws, F, and F.

To produce a good gauge of this class, the radius bar itself should be of steel, and carefully smoothed, throughout its length; and the scriber-holders also, denoted by H, and H, should be of steel or of close-grained iron. The slots in the holders are carefully fitted to the bar, after the bar is smoothed to the proper width and thickness. Whether the holders fit tightly or loosely on the bar is of little consequence; but it is necessary that the whole of the slot surfaces be smoothed and polished, that the holders may be easily moved along the bar without being liable to stick. The scribers are of hardened steel wire, and fixed to the holders by making a screw upon one end of each scriber and screwing it into a screwed hole in the bottom of each scriber-holder. Another mode of attaching the scriber consists in making a taper hole into the bottom of the holder, instead of making a screwed hole ; and into the taper hole the scriber end is smoothly fitted, so that it will admit of being gently driven in, to properly fix it, and also easily pulled out when it requires hardening and grinding.