Gauge, or GAGE, an apparatus for measuring any special force or dimension; thus we have pressure-gauge, wind-gauge (see ANEMOMETER), Rain-gauge (q.v.), wire-gauge, button-gauge, &c. The simplest form of gauge of dimension is the common wire-gauge, by which the diameter 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 gauges—the Birmingham gauge for iron-wire, sheet-iron, and steel differing from that used for brass, silver, gold, &c.; 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, hundredths, thousandths, or still smaller fractions, if necessary, be used, and that these be used for all diameters and thicknesses, such as wires, sheet-metals, buttons, watch-glasses, &c.; but such a scale has not yet come into general use. The Birmingham wire-gauge has, however, been widely adopted. The gauge commonly used for buttons and such like larger 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 placed between a and b (fig. 1), and the width of this space is measured by graduations on the middle metal slide.
A very elegant and delicate gauge is used 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, as ab and cd (fig. 2). Now, let us suppose the angle to be such that the distance between a and c is 2 inches, and that between b and d is 1 inch, while the lengths ab and cd are 10 inches. It is evident that for every inch of descent from a and c towards b and d there will be a narrowing equal to th of an inch; and for every tenth of an inch of such descent there will be a narrowing of th of an inch, and so on: thus we may, by graduating downwards from ac to bd, 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 fine screws with large graduated heads, Messrs Whitworth have measured small pieces of steel to the one-millionth of an inch (see MICRO-METER). Pressure-gauges, wind-gauges, &c. will be treated under the special subjects.—In railways, the gauge means the distance between the rails (see RAILWAYS).—The term GAUGING refers specially to the gauging of the contents of casks; and an excise officer (as gauging casks containing excisable liquors) is often called 'gauger.'