. The strength of materials; a text-book for engineers and architects. the effect that the movement of the sighted markis double the extension of the test-piece. The length of themicroscope is adjusted so as to give a constant adjustment should be tested with the extensometermounted on the specimen, and if necessary the length ofthe microscope tube can be altered by moving out or in theportion carrying the eye-piece. A complete revolution of thescrew L, which has a pitch of -V of an inch, should cause adisplacement of the mark through 50 divisions of the eye-piecescale, and
. The strength of materials; a text-book for engineers and architects. the effect that the movement of the sighted markis double the extension of the test-piece. The length of themicroscope is adjusted so as to give a constant adjustment should be tested with the extensometermounted on the specimen, and if necessary the length ofthe microscope tube can be altered by moving out or in theportion carrying the eye-piece. A complete revolution of thescrew L, which has a pitch of -V of an inch, should cause adisplacement of the mark through 50 divisions of the eye-piecescale, and when^this is the case the eye-piece is at the proper 376 THE STRENGTH OF MATERIALS distance from the objective. Readings are taken to tenthsof a scale division, so that this displacement, which would alsobe given by ^i^ of an inch extension of the test-piece,corresponds to 500 units. Each unit then means ^,.;.,,,. inchin the extension of the test-piece. A small extensometer based upon the same principle isused for measuring the compressive strain in short Fig. 171.—Ewings Extensometer. Dabwins Extensometer.—This instrument has beendesigned by Mr. Horace Darwin, , and is characterisedby simplicity and solidity of construction, which make itsuitable for heavy use. Another feature is that if the specimenshould break unexpectedly when the extensometer is affixedlittle damage will result. The instrument is made in two separate pieces each of whichis separately attached to the test-piece m, Fig. 172, by hardsteel conical points P, P and p, p. The steel rods carryingthese points are mounted in] slides and after being driven __ THE TESTING OF MATERIALS 377 gently into the centre-punch mark in the test-piece areclamped in position by the milled heads r, r. Both parts ofthe instrument should be capable of rotating quite freely aboutthe points, but there must be no backlash. The lower piece carries a micrometer screw fitted with ahardened steel point x and a divided
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyorkdvannostran
