Wire Torsion

Abstract

THE phenomena described by Major Herschel in his letter to NATURE, vol. xxii. p. 557, and about which he asks for information, are, we think, quite easily explained by what is known of the fluidity of metals. Yielding, or flowing, seems to occur in all metals after a certain limiting stress has been reached; indeed it probably occurs, although perhaps to an immeasurably small extent, even with small stresses (see Proc. Roy. Soc. No. 204, p. 411, 1880); but there is generally a limiting stress beyond which the increase of strain due to yielding becomes comparable in magnitude with the ordinary strains, which instantaneously disappear on the removal of the load. The bell-smith pulls his copper wire, and makes it much longer before he thinks it fit for use; in a similar way the telegraph constructor stretches, or kills the iron wire before he erects the line. Up to a certain limit of pulling force, the wire obeys the well-known laws of elasticity; slightly above that limit there is considerable fluid-yielding, there being but very little yielding below that limit; and at any instant during the lengthening if the man ceases to pull, the wire shortens a little. In fact at any stage the wire obeys the elastic law for small stresses. Eventually the man ceases to pull, knowing that the metal has lost most of its fluid properties, which can only be restored to it by annealing. The same thing occurs in brass, although to a smaller extent than in copper, which can be experimentally proved in the following way:— Stretch a piece of well-annealed brass wire in the manner described by Major Herschel until it is nearly breaking; and immediately set the wire vibrating. Now the note given out by the stretched brass wire, which, as is well known, depends on the tensile stress, will be found rapidly to go down in pitch. If the wire be tightened up again sufficiently with the screw, the original note will again be heard, and the pitch will again go down, but not so rapidly as before. Repeat this process until no flattening of the note is heard; then in this state we think that the experimenter will find the wire to break with much less torsion than before, and to obey Hooke's law more exactly. If it be desired to repeat the yielding or flowing process, the wire must be previously again annealed.