GAUGE, an apparatus for measuring any special force or dimension; thus we have pressure-gauge, wind-gauge (see ANEMOMETER), rainsange (q.v.), wire-gangs, button gauge, etc., simplest form of gauge of dimension is the common wire-gauge, by which the iameter of wire is measured. It is simply an oblong plate of steel, with notches of different widths cut upon the edge; these are numbered, and the size of the wire is determined by trying it in the different notches until the one is found which it exactly fits. The thickness of sheet-metal is tried by the same gauge. There is a great want of uniformity in these gr uges—the Birmingham gauge for iron-wire, sheet iron, and steel differing from that used for brass, silver, gold, etc.; and these again from the Lancashire gauges. It has been proposed, in order to obtain uniformity, and to enable definite descriptions and orders to be given with accuracy and certainty; that instead of the arbitrary numbers of varying• signification now in use, decimal parts of an inch, tenths, hundre.dths, thousandths, or still smaller fractions, if necessary, be used, and that these be used for all diameters and thicknesses, such as wires, sheet-metals, but tons, watch-glasses, etc.; but such a scale has not yet come into general use. The garage commonly used for buttons, watch-glasses, and such like large diameters, is a rule.with a groove cut lengthwise down the middle. Another metal rule, with a brass head, slides in this, and by means of a thumb-pin, may be pushed out at pleasure.
The object to be measured is ploced between the inside of the slide and the end of the rule, and the width of this space is measured by graduations on the middle metal slide.
A very elegant and delicate gauge has recently come into use for measuring watch glasses, and is applicable to many other purposes. On an oblong piece of sheet-metal. two straight metal ridges are fixed in such a manner that they shall be inclined at a given angle to each other. • Now, let us suppose the angle to be such that the distance between the upper extremities is 2 in., and that between the lower ends is 1 in., while the lengths of the metal ridges are 10 inches. It is evident that for every inch of descent from the upper to toward the lower ends, there will be a narrowing equal to of an inch; and for every tenth of an inch of such descent, there will be a narrowing of of an inch, and so on: thus we may, by graduating downwards from the top, measure tenths by units, hundredths by tenths, and so on to still finer quantities if required. This is applicable to lengths as well as diameters. By means of line Serews with large graduated heads, Messrs. Whitworth have measured small pieces of steel to the one millionth of an inch (see MicitolLETER). Pressure-gauges, wind-gauges, etc., will be treated under the special subjects.—In railways, the gauge means the distancebetween the rails. See ItamwAy.