. The Philosophical magazine; a journal of theoretical, experimental and applied physics. thelaw of F. Kohlrausch*,—<^0/<:Z<; = a^/^„, where « and n areconstants. In case of small torsions or short times, —d]dt= a(j)/t suffices. Thus for the purpose of deducing the time-rates, K, of increase of solid viscosity, it is permissible to re-gard the curves of fig. 3 as essentially linear. This is done in the following table. Table VIII.—Mean time-rate of increase of the absoluteViscosity of Steel, hard and soft. Table VI. Hard Steel. Twist Ko. 4-5 kXIO (/,es-. 220 140 240 230 140
. The Philosophical magazine; a journal of theoretical, experimental and applied physics. thelaw of F. Kohlrausch*,—<^0/<:Z<; = a^/^„, where « and n areconstants. In case of small torsions or short times, —d]dt= a(j)/t suffices. Thus for the purpose of deducing the time-rates, K, of increase of solid viscosity, it is permissible to re-gard the curves of fig. 3 as essentially linear. This is done in the following table. Table VIII.—Mean time-rate of increase of the absoluteViscosity of Steel, hard and soft. Table VI. Hard Steel. Twist Ko. 4-5 kXIO (/,es-. 220 140 240 230 140 Soft Steel. Mean rate K 194x10^ Mean viscosity at 600 sec. 140x10^ Table Table VII. Twist No. I icXlO ff/cs^.120014001900150012001000 Mean rate k 1400x10* Mean viscosity at 500 sec. 570 X10 Kohlraiisch, Iogg. A^ui. cxxviii. p. 21*) (1866). Viscosity encountered on passing from Fluid to Solid. 353 Y\fr 3.—The Absolute Viscosity of Hard Steel (Tables IV., VI-) and of (Tables V., VIL), considered in its variation with the timeelapsiiifj after lUUU ^UOtJ 3W0 U iUUU 2WU 30UU 354 Absolute Viscosity of Solids and Liquids. The general explanation of these results has been indicatedin § 11. In a liquid or a viscous fluid under moderate stressthe instabilities are supplied by the mere thermal agitation atordinary temperatures, at the same rate in which they areused in promoting viscous motion. Hence -sdscosity is con-stant at a given tem])erature. In a solid under stress theinstabilities are expended at a rate decidedly greater than thesmall rate of continuous supply. Thus viscosity decidedly in-iucreases with time. Suppose, therefore, a solid of initiallygreater viscosity starts on its viscous deformation at an instantwhen the viscosity of a solid of initially smaller viscosity hasbeen increased by time to the initial value of the former can actually be realized by twisting a soft steel rod aboutan hour later than a hard steel rod (tig. 3
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