120 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
specific resistance is decreased by moderate strain but increased by excessive strain; 
whilst with copper, moderate stfain effects increase, and excessive strain, decrease of 
resistance. 
The Effect of Temporary Stress on the Alteration of Electrical 
Resistance produced by Change of Temperature. 
As permanent strain had been proved to alter the susceptibility to change of 
resistance from change of temperature in a manner which suggested an intimate 
relationship between this susceptibility and the thermo-electric properties of metals, 
some attempts were made to determine the effect of such temporary stress as could be 
caused by longitudinal traction, on the alteration of resistance produced by change of 
temperature, partly with a view of establishing a still closer relationship between the 
above-mentioned physical qualities, and partly with the object of ascertaining whether 
the increase of elasticity which Wertheim’s experiments* seemed to have proved to 
be produced in iron and steel when the temperature is raised from 20° C. to 100° C., 
would be rendered manifest in experiments on the influence of stress on the electrical 
resistance. The difficulties here encountered seemed at first sight to be so great as to 
render it an almost impossible task to measure with any approach to accuracy the 
effect sought, unless this effect should be something very appreciable compared with 
the percentage alteration of resistance produced by stress at ordinary temperatures; 
for, as we have already seen, the increase of resistance produced by raising the tem¬ 
perature to 100° C. would be some hundreds of times greater than any change of 
resistance which can be produced in most metals by mechanical stress. After several 
failures, however, these difficulties were overcome, and I succeeded in measuring with 
almost the same accuracy the comparatively minute alterations of resistance produced 
by temporary longitudinal traction at the temperature of 100° C. as at the ordinary 
temperature of the room. 
To accomplish the desired object the large air chamber used in the first part of 
the enquiry was provided with two tubes, one near the bottom and the other near the 
top ; these tubes served the purpose of conveying steam into the bottom of the outer 
of the two cylindrical chambers of which the vessel consisted, and out again, near the 
top of the vessel, into a tub of cold w r ater, a hole at the top of the chamber which 
had been used previously for the purpose of filling the chamber with water having 
been corked up after the water had been emptied out. The air chamber was wrapped 
round with several layers of baize, and these in turn surrounded with several layers of 
paper tied round with string. The top of the air chamber was also well covered in the 
same manner with baize and paper, and the table on which the air chamber rested was 
surrounded on all sides with like material with the exception of a small aperture 
* Ann. cle Cliimie et de Pliys., 3 me serie, 1844, p. 431. 
