288 W. A. Norton— Experiments on Wood, Iron, and Steel bars. 



cross strain not long before, was generally in a condition to 

 suffer a greater deflection than it had before experienced under 

 the same load. The same was true of the steel bar during 

 several successive days of experiments with loads of 4 lbs. and 

 6 lbs. ; but as the result of these repeated strains the bar came 

 eventually ta be in a condition in which each renewal of the 

 stress gave, for the most part, a less and less deflection. 



10. It is apparent from the foregoing experimental results, 

 that every application of a transverse stress to a bar must induce 

 some change in its molecular condition, which continues, with 

 variations that may be either progressive or fluctuating, for a 

 greater or less interval of time. The duration of sensible in- 

 fluence varies with the amount and duration of the stress. For 

 the smaller strains it is but a few minutes ; for the larger several 

 days. The prolonged influence of strains applied from day to 

 ^ fact that th 



apparent from the fact ■ 

 'oduce either the 

 ngly shown in the experiments with 



did not on different days produce either the same deflection 

 the same set. It was strikii 



the steel bar by causing the bar, to which loads had been repeat- 

 edly applied for several previous days, to rest on its opposite 

 side, and comparing the deflection and set with those obtained 

 immediately before the reversal. It was found that the deflec- 

 tion produced by 18^ pounds was j\ greater than the deflection 

 produced by the same weight just before the reversal ; and the 

 set obtained was now many times greater than before. The 

 deflection also now increased with a prolongation of the strain, 

 whereas it before decreased. Also the set now increased for a 

 considerable interval of time after the withdrawal of the strain, 

 whereas it before decreased. 



11. There was no discernible limit of elasticity, revealed by 

 the experiments, with either wood, iron, or steel. A perceptible 

 set obtained, with each materia], immediately after the stress 

 was removed, however small its amount, until the set fell below 

 the lowest possible determination of which the apparatus was 

 capable (viz : Toin» of an inch, as the experiments were ordina- 

 rily conducted.) To test the question still farther, the delicacy 

 of the measuring apparatus was largely increased, by the adapt- 

 ation of a device for magnifying the movements to be observed : 

 and it was found that the least perceptible immediate set was 

 still limited only by the capability of detecting, with the appa- 

 ratus, minute displacements. 



If we take for the limit of elasticity the condition of things 

 at which a permanent set is obtained, the case is different. Thus 

 it was found that the set which subsisted after the pine stick 

 (3 in. by 3 in. and 4 ft. long), had been loaded at its middle 

 with 200 pounds {j\ the theoretical breaking weight), eventually 

 passed off entirely. This was the case whether the stress was 

 momentary or prolonged, and whether it was applied but once 



