THE 



AMERICAN JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



Art. I.— TAe Viscosity of Steel and its Relations to Temperature • 

 by Carl Barus. 



This paper is to be restricted to a discussion of the relation 

 between torsional viscosity and temperature, as observed with 

 steel in different states of hardness. Some mention of the 

 effect of stress on the amount of viscous motion in solids is, 

 however, unavoidable ; and the experiments lead naturally to 

 the investigation of a more general method, by which the in- 

 stantaneous deformation and the gradual deformation produced 

 by stress may be coordinated. The data already show that 

 imperceptible gradations lead from the purely viscous defor- 

 mation which follows strains within the elastic limits, to the 

 sudden, permanent set which follows strains beyond the elastic 

 limits. 



Method of measurement. 



Method. — Given a continuous straight steel wire of length L, 

 to which a convenient rate of twist, r, has been imparted. Con- 

 sider two right sections whose distance apart is the unit of 

 length and let <p be the amount of viscous angular motion of 

 the first relative to the second, during the time t. The relation 

 between (p and t is necessarily complex ; but if t be taken suffi- 

 ciently small, the variation of <p within this interval will be 

 uniform. Suppose the wire to be adjusted vertically and pro- 

 vided with an index to register angular motion at a distance b 

 from its lower end. Then will the motion at the index due to 

 the viscous detorsion of two sections whose distance from the 

 lower end is x and whose distance apart is dx (af>&), during 



the time t be ^— -cpdx (1) 



Am. Jour. Scl— Third Series, Vol. XXXIV, No. 199.— July, 1887. 



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