In manufacturing modern tools and machinery, the thousandth of an inch is the usual limit of allowable error. A [micrometer caliper] measuring to this limit is here shown. The pitch of its screw is 40 to the inch, and the beveled edge of the screw-thimble is divided into 25 parts, so that motion from one division to the next takes the screw 1⁄25 of 1⁄40 of an inch, or 1⁄1000. By carrying refinement a step farther, 1⁄10,000 of an inch can be detected. The production of a screw such as this was simply impossible by the lathe as used almost up to the close of the eighteenth century, its operator holding in his hand a gouge or chisel. Of inestimable importance was Henry Maudslay’s invention of the slide-rest which firmly holds the tool, moving it automatically along the wood or metal being cut. See [illustration] on page 96. James Watt, as he endeavored to improve the steam engine, before the slide-rest was invented, was sorely vexed and thwarted by the ill-shaped containers for steam which served him as cylinders. Perhaps the chief task accomplished by the lathe has been its own improvement, so that to-day surfaces are readily cut by its tools accurately to within a thousandth part of an inch. Vastly beyond this feat was Professor H. A. Rowland’s production of a virtually perfect screw, which enabled him to rule on concave gratings 5.9 inches square, 110,000 lines with such precision that the error between any two of the lines is probably less than 1⁄3,000,000 of an inch. These gratings brought to view spectra much more extended and clear than those observable in a spectroscope, however powerful. The concave plates employed by Professor Rowland were made by Mr. John A. Brashear of Allegheny, Pennsylvania.
Measurement is greatly indebted to accurate means of enlarging the images of objects as viewed in the telescope or the microscope. Glass grinding tools are to-day so exquisitely contoured that a lens forty-two inches in breadth shows the image of a star as an immeasurable dot. It was in pressing together two lenses of very large and known radius that Newton measured the lengths of light-waves. With homogeneous rays, such as those of yellow light, the successive rings of light and darkness marked the points at which the intervals between his lenses were equal to half a light-vibration or any multiple thereof. Measuring these intervals, by noting their distances from the common centre of his lenses, he found the wave-length of the particular light he was studying.
Two lenses as pressed together by Newton.
Newton’s rings as produced in yellow light.
Interchangeability Old and New.
The cheap duplication of products, so wonderfully expanded of late years, had its germ long before the Christian era, when in Babylonia a builder first made bricks in a mold, and took care by careful measurement to keep to uniform dimensions in his output. Because any brick matched any other from the same mold, he introduced a new beauty and regularity in architecture, he made it easy to extend or repair a wall, a gateway, a battlement. So it was afterward with the tiles, also made in molds, which were laid as floors or roofs; and the piping, likewise molded, for water-supply or drainage. To-day when a housekeeper replaces her worn-out stove-linings, and a printer increases his stock of type, they enjoy a direct inheritance from the first molders of bricks and tiles, cups and bowls. In a modern factory vast sums are expended in producing the original patterns, molded or copied perhaps ten million times, so that their cost, in so far as represented in each manufactured hook or lever, is next to nothing. Much expense, also, is entailed in making the jigs which guide the tools used in lathes or milling machines to turn out the cases of voltmeters, or a complicated valve-seat. A jig may cost a hundred dollars and its use may require rare steadiness of hand, the utmost keenness of eye; all the while the operator’s wife, at home, avails herself of an aid based on the very same principle. What else is the paper pattern according to which she cuts out a collar, an apron, a baby’s bib?
In machinery the first introduction of an interchangeability of parts was by General Gribeauval, in the French artillery service, about 1765. He reduced gun-carriages to classes, and so arranged many of their parts that they could be applied to any carriage of the class for which they were made. These parts were stamped, not forged. The next step in this direction was taken in America and, as in France, its aim was to improve instruments of war. Eli Whitney, famous as the inventor of the cotton gin, secured a contract from the United States Government for 10,000 firearms. These he manufactured almost wholly by stamping. He introduced machinery for shaping and, as far as then feasible, the finishing of each part. He also employed a system of gauges, by which uniformity of construction was assured for every gun produced. Next came J. H. Hall, of Harper’s Ferry, Virginia, who in 1818 made every similar part of a gun of such size and shape as to suit any other gun, improving some details of importance.