318 



TABLES 311-338.— VISCOSITY OF FLUIDS AND SOLIDS* 



The coefficient of viscosity of a substance is the tangential force required to 

 move a unit area of a plane surface with unit speed relative to another parallel 

 plane surface from which it is separated by a layer of the substance a unit thick. 

 Viscosity measures the temporary rigidity it gives to the substance. 



Fluidity is the reciprocal of viscosity expressed in poises. Kinematic vis- 

 cosity is absolute viscosity divided by density. Specific viscosity is viscosity 

 relative to that of some standard substance, generally water at some definite 

 temperature. The dimensions of viscosity are ML' 1 !"' 1 . It is generally ex- 

 pressed in cgs units as dyne-second per cm 2 or poises. 



The viscosity of fluids is generally measured by one of several methods 

 depending on the magnitude of the viscosity value to be measured. For vapors 

 and gases as well as for liquids of low viscosity, measurements of viscosity are 

 made by the rate of flow of the fluid through a capillary tube whose length is 

 great in comparison with its diameter. The equation generally used is 



. . . yirqd 4 t /, mv 2 \ 



* the viscos,ty, = 128Q(/ + A) -^- — | 



where y is the density (g/cm 3 ), d and / are respectively the diameter and 

 length in cm of the tube, Q the volume in cm 3 discharged in t sec, A the 

 Couette correction to the measured length of the tube, h the average head in cm, 

 m the coefficient of kinetic energy correction, mv 2 /g, necessary for the loss of 

 energy due to turbulent, in distinction from viscous, flow, g being the accelera- 

 tion of gravity (cm/sec 2 ), v the mean velocity in cm/sec. (See Herschel, Nat. 

 Bur. Standards Techn. Pap. Nos. 100 and 112, 1917-1918, for discussion 

 of this correction and A.) 



For liquids of medium and high values of viscosity measurements are made 

 by Margule's method of observing the torque on the inner of two concentric 

 cylinders while the outer is rotated with constant angular speed with the vis- 

 cous liquid filling the space between, or by noting the rate of fall of a solid 

 sphere through the liquid. 



For the method of concentric cylinders the equation is 



, . . K6(R 1 2 -R, 2 ) 



,, the viscosity, = ^RS R 2 * L ' 



where K denotes the elastic constant of the torsion member supporting the 

 inner cylinder of radius R 2 cm and length L cm, 6 is the angular displacement 

 of the inner cylinder from its position of equilibrium, CI the angular speed of 

 the outer rotating cylinder of radius Ri cm in the corresponding units em- 

 ployed to measure 6. The necessary corrections due to end effects of cylinders 

 of finite length are given in the reference. 108 



For the falling sphere method, the equation is that of Stokes law as modified 

 by R. G. Hunter: 109 



. . . 2R 2 (d l -d.) 

 ■q, the viscosity, = g ~ 





where y denotes the radius in cm of the crucible containing the liquid of density 

 d 2 (g/cm 3 ), to a depth of h cm, R the radius in cm of the sphere of density d x 

 (g/cm 3 ), and V the velocity (cm/sec) of the falling sphere. 



* The data on viscosity were selected and arranged by George V. McCauley, Corning 

 Gluss Works 



108 Lillie, H. R., Journ. Amer. Cer. Soc, vol. 12, p. 505, 1929. 



,ou Hunter, R. G., Journ. Amer. Cer. Soc, vol. 17, p. 123, 1934; Ann. d. Phys., ser. 4, 

 vol. 22. p. 287, 1907; vol. 23, p. 447, 1907. 



SMITHSONIAN PHYSICAL TABLES 



