Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 242]

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234 to return to original condition after elongation due to strain. Strength may therefore be represented in various ways: that is, it may be represented in units of weight necessary to rupture, or units of weight necessary to produce any stretch in percentage of original length. Elasticity may in general be fairly represented in the percentage of stretch suffered previous to rupture. But the stretch may be of two kinds. If, for instance, a fibre be submitted to strain not sufficient for rapture, it will stretch. If this strain be removed the tendency will be to return to its original length, but this return will be incomplete. The fibre will have permanently stretched, and will have set. The difference between the total stretch and this permanent stretch constitutes the elastic stretch. It is upon data of this kind, and the relation between them, that we must depend for an appreciation of the ultimate value of wool. These data with regard to wool are obtained in the following mann e r : A dynamometer is constructed in which a wheel is delicately mounted to avoid friction as far as possible by pointing the extremities of its axle,* and inserting these in conical boxes, so that the wheel is really supported upon points. Fixed to the axle is a pendulum, or lever, weighted at its lower extremity. Over the periphery of the wheel passes a light chain, which supports at one «end a screw clamp. Underneath this clamp, and in the same vertical plane, is a second clamp fixed to a rod, which may be moved cip and down by a screw motion at the base of the instrument. Attached to the clamp is a horizontal indicator, the point of which during the motion of the clamp may pass over a scale engraved upon a frame supported by the upper clamp. The frame bears upon one arm a scale graduated to millimetres, and upon the other arm a scale graduated to one-fiftieth of an inch. The end of the pendulum, or lever, may move over a scale upon an arc graduated to grammes and fractions of grammes by experiment. To test the strength and elasticity of a fibre it is fixed in the clamps, which are exactly 20 millimetres apart. By means of the screw motion strain is very gradually applied, and the pendulum moves from the vertical and furnishes the resistance. As the strain increases the fibre stretches, the clamps become more widely separated, and the degree of separation is measured by movement of the indicator attached to the lower clamp over the scale upon the frame attached to the upper one. To secure a fair average for wool 30 fibres must thus be tested from each sample. Now the determination of the ultimate strength and elasticity may be made in two ways. First, and in order to record the true elasticity, a certain strain is applied,, say sufficient to produce elongation of one millimetre. The strain is then relieved, and the fibre allowed to regain as far as possible its original length. When the contraction appears to be complete the strain is applied, and the total stretch and the permanent stretch are recorded. Strain is again applied until stretch of two millimetres is produced, when it is relieved and the contraction observed. Such experiment is repeated until rupture is effected. In the record, therefore, we have strain, total stretch, and permanent stretch or set. It is plain that the difference between the total stretch and permanent stretch or set represents what we ordinarily understand to be elasticity.