the Limit of Elasticity in Metals, 195 



to enable the reader to judge of the trustworthiness of my ob- 

 servations, I will describe the apparatus and the mode of obser- 

 vation which I adopted, limiting myself to what is most essential. 



§ 2. Mode of Observation. 



The kinds of steel and iron which I used were prepared from 

 Swedish ores, in the form of rolled, partly round and partly 

 rectangular bars 6 feet in length ; the round ones were half an 

 inch in diameter, and the square ones half an inch in the side. 



The stretching-experiments were made with the Fuller's 

 machine* used by Lagerhjelm in the year 182$, in which 

 the bars were fastened between an hydraulic press on the one 

 hand, and a lever with unequal arms on the other; the latter 

 increased the load on the scale twentyfold. To measure the 

 lengthening of the bars tested, a couple of small finely graduated 

 scales were used, the " Index " scale and the "Measuring" 

 scale, which were screwed on the bars at a distance from each 

 other of 5 Swedish feet. Above, on each scale, a powerful micro- 

 scope was fixed on a large pair of wooden beam compasses, 

 which by means of a fork was firmly fixed with its ends against 

 the scales, and therefore against the bars under examination. 

 Its weight was almost entirely got rid of by suitably adjusted 

 compensatory weights. There was also an arrangement by 

 which the compass was compelled to follow the index-scale, so 

 that a small deviation was sufficient to place the index-micro- 

 scope on the zero-point: the real measuring was effected by 

 means of the micrometer-screw on the microscope above the 

 measuring-scale. 



A division of the measuring -scale was almost exactly 0*2 mil- 

 lim. ; a turn of the micrometer -screw almost a quarter of a divi- 

 sion, or 0*05 millim. The direct reading of the milled head 

 went to hundredths of a turn, or - 0005 millim. It is thus seen 

 that the accuracy of the apparatus is far greater than what is 

 required. 



As far as the excellence of the arrangement is concerned, it 

 may be mentioned that the distance of the parallel hairs in 

 each microscrope is about 0*7 of a turn of the screw, or 0*04 

 millim.; according to which, the sum of the errors made in 



* These experiments were instituted in the summer of 1862, on behalf 

 of the committee which had for its object to ascertain the applicability of 

 Swedish iron and steel as material for railways. 



The fitting up of this machine, and the essential arrangement of the 

 measuring-instruments used, were effected by the care of M. C. A. Ang- 

 strom, who also began the breaking-experiments which I afterwards con- 

 tinued. The experiments on elasticity proper were mostly made by myself, 

 but in part by Engineer K. W. Cronstrand. 



02 



