Stress and Strain on the Properties of Matter, 103 



apparently very slightly increase the elasticity after recent permanent 

 extension. 



Several other experiments of a like nature were made with annealed 

 iron, but all seemed to show that the elasticity as tested by the method 

 of longitudinal vibrations was not appreciably altered permanently by 

 permanent extension. 



i Similar experiments to the above were made with annealed copper 

 wire, and with similar results, except as regards the effect of rest, 

 which in this case produced no change. 



A Comparison of Moduli of Torsional Elasticity as determined bij the 

 Statical and Kinetical Methods. 



Statical Method. 



The wire to be tested, about 28 feet in length, was fastened at its 

 upper extremity to a clamp secured to a stout iron bracket. The lower 

 extremity of the wire was clamped at 0, fig. 2, Plate 2, the extremity 

 of a brass rod J inch in diameter,, and 1 \ feet in length ; the rod passed 

 vertically through the centre of a horizontal brass plate P, 8 inches in 

 diameter, so that half of the rod was above and the other half below 

 the plate ; the lower half of the rod terminated in a hook to which 

 was suspended a scale-pan, S, weighing 2 kilos. The torsion was 

 effected by placing weights* in two scale-pans, T, made of cardboard, 

 and weighing 10 grams each ; the scale-pans, T, were fastened each to 

 one extremity of a light silk thread, which passed over a pulley, W, 

 and was wrapped a few times round the rod ; the two threads were 

 wrapped round the rod in opposite directions, so that when equal 

 weights were placed in the scale-pans, T, a torsional couple was pro- 

 duced. The plate P was divided at its circumference into degrees, 

 and by using a fine steel pointer (fig. 3), placed above the rim of the 

 plate, and nearly but not quite touching it, it was possible to estimate 

 to one-tenth of a degree. The stands carrying the pullies, W, were 

 capable of being moved either vertically or horizontally in any direc- 

 tion, and great care was taken to ensure that the parts of the silk 

 threads between the rod and the pullies were parallel to each other. 

 In order to accomplish this last the adjustments were made in the first 

 instance as nearly correct as the eye could judge ; torsion was now 

 imparted in the manner described, and after the plate had been 

 twisted through about 360 degrees, the position of the end of the 

 pointer, as regards its distance from the rim of the circular plate, was 

 noted. If this distance was not the same as before the application of 

 the torsion, one or both of the pulley stands were shifted in a hori- 

 zontal plane until there was no perceptible difference in the distance 



* These weights were of thin strips of German silver and were each made very 

 accurately equal to 10 grams. 



VOL. XLIII. I 



