Now adjust the tongue of the caliper to the bottom of this plug (as shown in the engraving) and the vernier should read 1.625 minus one-half the diameter of the plug, or 1.500, and any variation from this will show the error of the jig. In this case the top surface of _B_ was used and no allowance had to be made for its thickness. In case the bottom surface is used, 0.150 must be deducted from the reading of the caliper. [Illustration: Fig. 16. Attachment for Vernier Calipers] It is very easy to make a mistake in setting a bushing, and such a mistake is equally hard to detect unless some such means of measuring as this is at hand. It often happens that jigs and fixtures are put into use containing such errors, and the trouble is not discovered until many dollars' worth of work has been finished and found worthless. The illustration shows but one of the many uses to which this attachment may be applied. The figures given on the details are correct for making an attachment to be used upon the Brown & Sharpe vernier caliper, but for other calipers they would, of course, have to be altered to suit.[7] Improved Micrometer Beam Caliper [Illustration: Fig. 17. Improved Micrometer Beam Caliper] In a beam caliper having a sliding micrometer jaw with or without a separate clamping slide, it is necessary to have the beam divided into unit spaces, at which the jaw or slide may be accurately fixed, the micrometer screw then being used to cover the distance between the divisions; but it is difficult to construct a beam caliper of this type with holes for a taper setting pin, at exactly equal distances apart; consequently a plan that is generally followed in making such tools is to provide as many holes through the slide and beam as there are inch divisions, each hole being drilled and reamed through both the slide and beam at once. If it were attempted to drill the holes through the beam at exactly one inch apart, having only one hole in the clamping head and using it as a jig for the purpose, it would be found very difficult, if not impossible, to get the holes all of one size and exactly one inch apart. The design of the micrometer beam caliper shown in Fig. 17, which has been patented by Mr. Frank Spalding, Providence, Rhode Island, is such, however, that it is not necessary to drill more than one hole through the clamping slide. The beam _F_ is grooved longitudinally, and in the groove are fitted hardened steel adjusting blocks in which a taper hole _D_ is accurately finished. Between the blocks are filling pieces _G_, which are brazed or otherwise fastened in the groove. Holes are drilled, tapped, and countersunk between the blocks and the filling pieces _G_, in which are fitted taper head screws _EE_1_. The construction is thus obviously such that the blocks may be shifted longitudinally by loosening one screw and tightening the other. In constructing the caliper, the holes through the beam are drilled as