565. 



was now tried. Steel wire of ^Lth of an inch in diameter was 

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

 heated to redness. Then, after 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*35 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 wire 

 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 235 turns of wire, -^ of an 

 inch in diameter, weighing altogether 230 grains. Unstretched, 

 it measured 6''7 inches, but with a weight of 1251 grains attached 

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

 with it showed an elongation of '00 157 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. Kupffer's re- 

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

 Rendu Annuel, St. Petersburgh, 1856). He finds by his method of twisting and 

 transverse oscillations, that the decrease of elasticity for steel and copper is '000471 

 and -000478. Very careful experiments recently made by Prof. Thomson, indicate 

 a si ght increase of expansibility by heat in wires placed under tension. August 1. 

 J. P. J. 



