In this place measuring tools are considered to include such implements as divided measures, rulers, and callipers, and also tools for ascertaining qualities and conditions of various pieces of work: to this class of tools belong straight-edges, squares, gauges of several sorts, and surface-plates.

Straight-Edges

A straight-edge is generally a steel or wood right-angled parallelopiped, whose length is twenty or thirty times its width, and two or three hundred times the thickness. The two broadest surfaces which extend along the length of the instrument are termed its sides, and the two smaller surfaces that extend along the length are named the edges, and these consist of two planes that are parallel to each other. These two edges are the only portions of the implement that can be referred to when applying the tool to use; consequently it is named a straight-edge, and sometimes a ruler. Such tools are made of all lengths, from three inches, to fit a waistcoat-pocket, to several feet or yards. Straight-edges are much used to mark straight lines upon work, and also to ascertain if some portion of a surface is a plane, or how near it resembles a plane. The primary use of the tool is to indicate straight lines and lengths, and these are the elements of precision in measurements ; therefore a straight-edge is the first tool to be made by a beginner, when he has so far advanced in his work that precision is his object. The simplest sort of straight-edge is that indicated by Fig. 284, being without a divided measure, but having curved ends to admit a free handling, and to render it fit for carrying in a pocket, if necessary. Fig. 285 denotes a ruler whose length is sixteen centimetres, each centimetre being marked on one of the broad surfaces of the ruler. Fig. 286 represents a ruler whose length is shown to be sixteen centimetres, and which is also shown to be one hundred and sixty millimetres in length, this length being the same as sixteen centimetres, because one centimetre equals ten millimetres. From the left-hand extremity of the ruler to number 10 is the length of one decimetre, or, which is the same length, ten centimetres, as indicated on the straight-edge ; and if the instrument were ten times the length of ten centimetres, it would be one metre in length, and therefore would contain one thousand millimetres, because one hundred millimetres equal one decimetre. During the application of any straight-edge to a piece of work, it is necessary to so place the tool that its two broad sides shall be at right angles to the surface to which the straightedge is put; if not, one of the corners of the tool will touch the work, at which time the tool will bend through its own weight, and its utility is, for the moment, destroyed.