Thermal Effects of Lonyitudinal Compression of Solids. 539 



line was given in the Phil. Mag. for September, 1867, the author 

 has verified the theory of Professor Thomson, as applied to the 

 thermal effects of laying weights on and taking them off metallic 

 pillars and cylinders of vulcanized india-rubber. Heat is evolved 

 by compression, and absorbed on removing the compressing force 

 in every substance yet experimented on. In the case of metals, the 

 results agree very closely with the formula in which e, the longi- 

 tudinal expansion by heat under pressure, is considered the same 

 as the expansion without pressure. It was observed, however, that 

 all the experimental results were a little in excess of the theoretical, 

 and it became therefore important to inquire whether the force 

 of elasticity in metals is impaired by heat. In the first arrange- 

 ments for this purpose, the actual expansion of the bars employed 

 in the experiments was ascertained by a micrometric apparatus, — 1st, 

 when there was no tensile force, and 2nd, when a weight of 700 lbs. 

 was hung to the extremity of the quarter-inch rods. The results, 

 rehable to less than one-hundredth of their whole value, did not 

 exhibit any notable effect of tensile force on the coeificient of ex- 

 pansion by heat. An experiment susceptible of greater delicacy 

 was now tried. Steel wire of g'oth of an inch in diameter was 

 wound upon a rod of iron ^ of an inch in diameter. This was 

 heated to redness. Then, alter plunging in cold water, the spiral 

 was slipped off. The number of convolutions of the spiral was 420, 

 and its weight .58 grains. Its length, when suspended from one 

 end, was 6 '3.5 inches, but on adding to the extremity a weight of 

 129 grains, it stretched without sensible set to 14"55 inches. The 

 temperature of the spiral thus stretched was raised or lowered at 

 pleasure by putting it in, or removing it out of an oven. After 

 several experiments it was found that between the limits of tempe- 

 rature 84° and 280° Fahr., each degree Centigrade of rising tem- 

 perature caused the spiral to lengthen as much as -00337 of an 

 inch, and that a contraction of equal amount took place with each 

 degree Centigrade of descending temperature. Hence, as Mr. James 

 Thomson has shown that the pulling out of a spiral is equivalent 

 to twisting a wire, it follows that the force of torsion in steel vrire 

 is decreased -00041 by each degree of temperature. 



An equally decisive result was obtained with copper wire, of 

 which an elastic spiral was formed by stretching out a piece of 

 soft wire, and then rolling it on a rod i of an inch in diameter. 

 The spiral thus formed consisted of 23.5 turns of wire, J„ of an 

 inch in diameter weighing altogether 230 grains. Unstretched 

 it measured 6 7 inches, but with a weight of \2i)l grains attached 

 to it, it stretched, without set, to 10'05 inches. Experiments made 

 with it showed an elongation of "00157 of an inch for each degree 

 Centigrade of elevation of temperature, and an equal shortening on 

 lowering the temperature. The diminution of the force of torsion 

 was in this case •00047 per degree Centigrade*. 



* Since writing the above, I have become acquainted with M. Kiipffer's re- 

 searches on the influence of temperature on the elasticity of metals (Compte 

 2 N2 



