MEMOIRS

OF THE

LITERARY

AND

PHILOSOPHICAL SOCIETY

OF

MANCHESTER.

It is well known, that if a body be stretched with a force, such as but little to exceed 1/3 of its ultimate strength, its elasticity will remain perfect ; but if the force be greater, as 1/2 or 2/3 of the full strength, the elastic force will be impaired, and the length of the body somewhat increased, as is shown in the following experiment. I took a thin wire 20 feet long, and fastenind it firmly at one end to an elevated object, let it hang vertically with a small weight attached to the other end to render it straight : then, hanging weights at the bottom, increasing by 10 lbs . at a time, and always taking the preceding weights off before the others were laid on, I found that the wire constantly returned to the same length, till 90 lbs. had been laid on, when it showed an inappreciable increase of length : with 100 lbs. the increase was .03 inch, with 140 lbs. it was .2 inch, and the wire broke with 208 lbs. The ratio of the elastic to the breaking force was therefore 90/208 = 3/7 nearly.

To see what defect in strength a wire would exhibit when broke several times successively I took a piece of the same thickness as that above, and suspending it in the same manner as before, broke it six times by weights hung at the bottom, and increased about 4 lbs. each time ; the results are as below : -

The wire, exclusive of the fastenings, was 8 feet long when the experiments began, and 3 feet 3 inches when they ended. With another wire of the same thickness, broke six times successively, the results were as below : -

From these experiments it appears, that stretching a wire frequently, even to its breaking point, does not materially reduce its power of bearing again. The case in a bolt may not be exactly the same, as in a thin wire, the manufacture of the two being different ; but it does not seem probably that any great difference would arise from that cause.

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