THE MEASUREMENT OF STRESS BY THERMAL METHODS. 245 
possible generally to trace the relation of stress to strain and thermal change for pure 
compression stress up to the point where the departure from Hooke’s law was very 
definitely marked ; but sufficient work was accomplished to show that the strain and 
thermal change are both proportional to the stress throughout the greater part of the 
elastic range, and it seems highly probable, from the evidence obtained in the tension 
experiments, that this will hold throughout. the whole elastic range of stress, as deter- 
mined by strain measurements. 
In order to indicate the nature of the results the following experiment may be 
quoted. The specimen was of wrought-iron, 0°9 x 0°39 inches in section and 4°5 inches 
long. The strain-measuring instrument used was one specially designed by Professor 
Hwine for compression, and similar in principle to the extensometer used in a previous 
experiment, except that the distance between the grips was 1°25 inches, and there was 
no calibrating screw. The instrument was first calibrated on a Whitworth measuring 
machine, and the position of the micrometer eye-piece determined, so that one division 
of the scale corresponded exactly to 355)9p Of an inch. The instrument was then set 
up on the specimen, and the thermopile applied to the broad face. The specimen was, 
for convenience, stressed in a small press, provided with an hydraulic diaphragm, 
accurately calibrated up to 21,000 pounds, and the load was applied as uniformly as 
possible. Preliminary experiments were made to obtain the correction factor for 
radiation and conduction, and the value of k was found to be 0096. A load was applied 
at the rate of 2000 pounds in ten seconds, until the specimen failed by buckling. The 
following readings were obtained in this way :— 
TaslE VII. 
anaes ond Compressometer Galvanometer ees 
i é Reading. Reading. k= ‘0096. 
0 0 0 0 0 
—58 
10 2,000 58 3:2 3-45 
— 60 
20 4,000 118 6-6 7:28 
—59 
30 6,000 177 9°8 11-20 
a57, | 
40 8,000 234 12°2 14:54 
— 60 
50 10,000 294 15°0 18°60 
— 58 
60 12,000 352 176 22°66 
— 58 
70 14,000 410 19°6 26°18 
Failed by bending 
Notes.—Long connecting leads. 
Distance of scale from galvanometer mirror = 10’ 2”. 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART II. (NO. 9). 37 
