MEASUREMENT An advance of the greatest importance made in mechanical engineering is that of measurement. Since the beginning of the 19th century steady movement has been going on in this direc tion. Methods of measurement adopted in woodworking have but little application in high-class engineers' work. They are adopted, however, to a considerable extent in the metal trades which are allied to engi neering, as sheet metal working, girder work, etc. When a carpenter or joiner sets about constructing a door, window sash, roof or box he takes a two-f oot rule, a marker and possibly compasses. Whether the tool used be saw, chisel, gouge or plane, the woodworker estimates by sight alone whether or not the lines marked are worked by.
The broad difference between his method and that of the en gineer's machinist lies in this, that while the first tests his work by the eye, the sec ond judges of its accuracy or otherwise by the sense of touch. It may seem that there cannot be very much difference in these two methods, but there is. To the first, the sixty-fourth part of an inch is a fine dimen sion, to the second one-thousandth of an inch is rather coarse. In what are called "limit gauges" the plugs and rings are made of slightly different dimensions. If a plug is made a thousandth of an inch less than its ring, it will slip through it easily with very perceptible slop. The common rule is therefore scarcely seen in modern machine shops, while the common calipers fill but a secondary place, their function having been invaded by the gauges. A minute dimension cannot be tested by lines of di vision on a rule, neither can a dimension which should be fixed be tested with high precision with a movable caliper of ordinary type. Yet it must not be supposed that the adoption of the system of gauging instead of the older methods of rule measurement relieves men of responsibility. The instruments of precision require deli cate handling. Rough forcing of gauges will not yield correct re sults. Without correctness of measurement mechanical construc tions would be impossible, and the older device of mutual fitting of parts is of lessening value in face of the growth of the inter changeable system, of international standards, and of automatic machine tools.
The two broad divisions of measurement by sight and by con tact are represented in a vast number of instruments. To the first
named belong the numerous rules in wood and metal and with English and metric divisions, and the scales which are used for setting out dimensions on drawings smaller than those of the real objects, but strictly proportional thereto. The second include all the gauges. These are either fixed or movable, an important sub division. The first embrace two groups—one for daily workshop service, the other for testing and correcting the wear of these, hence termed "reference gauges." They are either made to exact standard sizes, or they embody "limits of tolerance," i.e., allow ances for certain classes of fits, and for the minute degrees of inaccuracy which are permissible in an interchangeable system of manufacture. The movable group includes a movable portion, either corresponding with one leg of a caliper or having an ad justable rod, with provision for precise measurement in the form of a vernier or of a screw thread divided micrometrically. These may be of general character for testing internal or external diameters, or for special functions as screw threads. Subtitles indicate some particular aspect or design of the gauges, as "plug and ring," "caliper," "horseshoe," "depth," "rod," "end measure," etc. So severe are the requirements demanded of instruments of measurement that the manufacture of the finer kinds remains a speciality in the hands of a very few firms. The cost and experi ence necessary are so great that prices rule high.