Gap gauges are used for measuring diameters or thicknesses; and are made of thin plate iron or steel. One gauge may contain several gaps, if all the gaps are to greatly differ from each other in width, that no mistake may occur through measuring with the wrong gap; but when the gaps are very nearly alike, only two or three should be made in one gauge.

Fig. 239 indicates a gauge whose handle is smaller than the remainder, to admit an easy handling; and whatever may be the widths of the gaps required, the handles of all such gauges should be made large enough to be comfortably used ; therefore, in many gauges the handles are larger than the end which contains the gaps ; and in other gauges the gap-ends are of the same widths as the handle-ends. When a number of gaps are necessary, some of them should be at each end of the gauge, as denoted by Fig. 240, instead of all the gaps being at one end.

If gauges of this class are to be used for heavy work which is being reduced or flattened to a precise dimension with a steam hammer, it is necessary to make the gauges of sufficient length to prevent the workman who holds them being scorched with the heat from the work ; and each gauge may have a hole, that it may be hung in a place secure from injury ; also around each gap should be written the name of the particular work for which the gap was made.

During a long usage, the gaps in such gauges become worn too wide; the remedy for this is to heat the gap-sides and hammer them to make the openings of less width; after which, the gaps are enlarged to the exact width required, with a little filing.