A standard measure is any measure that may be adopted as a standard by any individual, whether he makes it himself or purchases it from those whose business it is to make standards. The particular standard rings and plugs selected by any one maker are those which suit his particular work. The standards that he selects to be used for measuring general work, are those of his own make, and several plugs and rings of one size are required in order that they all may be in use at one time when a great number of pieces of work of the same dimensions are in progress. Such gauges are made to resemble the standard which he adopts as his authority, and in England the Whitworth gauges are the standards in use at the present time. These are hard, and therefore not liable to wear with proper usage. Each one has a name that indicates the number of inches and parts in the diameter of its cylindrical portion, and the difference of diameter is the sixteenth, eighth, tenth, and other portions of an inch, as they are usually named. The most useful and universal dimensions for plugs and rings are those differing by millimetres, and of these two Tables are now given. Table 3 denotes the dimensions of the plugs, and in the first column are seen all the names of the plugs, each name denoting the diameter of the gauge; in the second column are seen the total length for each different diameter; the lengths of the cylindrical portions are given in the third column, and the lengths of the handles in the fourth column. In Table 4 are given the dimensions for each ring, the first column indicating the diameters of the holes, the second column showing their heighths or widths, and third column denoting the extreme diameter of each ring.

Table 3

Table 4

The smaller sizes of plugs have broad thin handles, which are larger than the cylindrical portions, for convenience of holding with a finger and thumb; their rings also are comparatively large; such are represented by Fig. 518. Plugs about thirty or forty millimetres thick have handles about as thick as the cylindrical portions; these are denoted by Fig. 519. The heavy gauges require holes through their handles to hold a lever for moving the plugs and for lifting them; the handles of large plugs are smaller than the parallel portions, and are denoted by Fig. 520.

In order to allow the rings to be easily and quickly tried upon the work which is in progress, the entrances to the holes are curved and smoothly polished; both ends of the hole are thus treated to permit the ring to be placed either way first on the work being tried. Each plug is also curved at the end, for effecting an easy entrance into the gauge ring, and for entering any other hole for which the plug may be used as a standard. To properly use ring and plug gauges it is necessary to make the surface of the work to be measured as smooth as possible previous to trying in the plug or trying on the ring; if the work is not smooth, it cannot be measured with any gauge whatever. Those who are particular in the management of cylindrical gauges keep them oiled with good oil, and in a box free from dust; and every time a plug or ring is to be put to a piece of work, both the work and gauge are first cleaned with a clean cloth, and then oiled with clean oil.