(229 ) 
1TX.—On the Measurement of Stress by Thermal Methods, with an Account of some 
Experiments on the Influence of Stress on the Thermal Expansion of Metals. 
By E. G. Coker, M.A. (Cantab.), D.Sc. (Edin.), F.R.S.E.; Assistant Professor of 
Civil Engineering, M‘Gill University, Montreal. (With Two Plates.) 
(MS. received April 11, 1904. Read June 6, 1904. Issued separately September 2, 1904.) 
CONTENTS. 
PAGE ; PAGE 
1. Introduction . : 229 6. The Relation of Stress to Strain and Thermal 
2. The Thermal Effect of Wencien il Chmapwas: Change in Short Compression Members . 248 
sion Stress. 231 7. The Variation of Compression Stress in a Long 
3. The Thermal Biopaneion lo Brass ‘and ‘Steel Compression Member . : 247 
under Tension Stress . 232 8. The Variation of Stress in the Chay Section a 
4. The Behaviour of Iron and Steel smiles Teasile a Beam . : 4 : : : ; . 248 
Stress. ; 238 9. Conclusion ? : : : ; : e250) 
5. The Relation of Stress to Strain al iene 
Change in Tension Members ‘ : » S24] 
1. INTRODUCTION. 
In the determination of the effects of stress upon different materials, the investigator 
has several methods of attack open to him, each of which has its own particular 
advantages. In the great majority of cases the material under investigation obeys 
the generalised Hooke’s law, and the effects of a stress are therefore most easily followed 
and measured “by observations of the strains produced. The strains being usually 
exceedingly minute, it is necessary to magnify them sufficiently to allow of accurate 
measurement. To this end many instruments have been devised for measuring the 
Strains obtained by the action of different stresses, and in fact the great majority of 
our experimental knowledge has been obtained in this way. The application of polarised 
light to the determination of stress was first suggested by Brewster,” and he applied it 
to many problems, particularly the determination of the neutral axis of a glass beam. 
Neumann,{ with a full knowledge of the work of Brewsrmr, developed a theory of the 
analysis of strain by polarised light, and Maxwe.u{ also independently developed a 
theory. A third method, which has assumed great prominence in recent years, is the 
microscopic examination of metals under stress, as developed by Ewrne, and RosEenHatn,]|| 
and others. 
The present paper is mainly concerned with the measurement of stress by the 
temperature changes produced, a subject to which attention was first drawn by WEBER, § 
who found that when a wire was stretched suddenly a thermal effect was produced, 
* Trans. R.S.E., vol. iii. 
t+ Abhandlungen der k. Akademie der Wissenschaften zw Berlin, 1841. 
£ “On the Equilibrium of Elastic Solids,” Trans. R.S.H., 1853. 
|| “On the Crystalline Structure of Metals,” Trans. R.S., 1900. 
§ Poggendorf?s Annalen, Bd. xx., 1830. 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART II. (NO. 9). 35 
