Fig. 353.—S—Block square.
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752.—Where butt rods are used for continuous measurement, it is necessary that they be brought very carefully together. In base line measurement three or four are used, but for metal work or masonry two 10-feet only are generally employed. It is necessary that the rods should lie upon a straight surface or be supported in a straight line. In bringing them together, a piece of indiarubber 1/8 inch or so in thickness temporarily placed at one end will prevent any palpable disturbance of the percussion if the fixed rod be well weighted. One 5-feet more fully divided butt rod is very commonly supplied with a pair of 10-feet rods for supplemental measurement.
753.—Angle-piece.—A solid angle-piece with two planes at right angles is very convenient for use with butt rods to give means of scribing the true length down to a surface, Fig. 353 S.
754.—Butt Rods with Iron Core.—Where rods are to be used for preparing iron work it is better to have an iron core through the rod, that may expand and contract with the metal on which they are used. The rods that the author has designed for this purpose are made out of a length of seasoned pine 2¼ inches square, sawn down and turned cut sides outwards to prevent warping. A 10-feet length of iron steam tube about ½ inch diameter is painted several times and then bound round with paper soaked in paraffin. This is placed in a pair of meeting grooves, as shown in section Fig. 355. The two pine flitches are cross-tongued together and glued up with the inserted tube between them. The tube has a turned steel cap placed over each end, Fig. 354, and this is ground in a lathe to true standard at the temperature of 60° Fahr. A steel pin is placed through the centre of the rod to indicate 5 feet. The finished size of the rods is 2 inches square.
The author has made these rods in sets, consisting of two 10-feet and one 5-feet packed together with an angle-piece, Fig. 353 S, in a deal case.
755.—The 5-feet Rule is of steel, ¾ inch by ¼ inch, inlaid in a piece of dry pine, altogether of only half the thickness of the rods, so that it stands the correct height for central butt measurement, Fig. 356. The rule is divided into feet and inches, with one foot to eighths. A centigrade thermometer is placed in one of the rods to indicate the prevailing temperature, and a small piece of scale showing amount to be allowed in 10 feet per degree centigrade for temperature above or below 15·5° centigrade is engraved upon the thermometer scale. The coefficient for the expansion of wrought iron is given by Lord Kelvin as ·000019 mean per degree centigrade.
Figs. 354, 355, 356.—Butt rods with iron core.
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