THE PHYSICAL PEOPERTIES OF INFECTIVE PARTICLES 233 



inner cylinder, causing it to turn through an angle until the restoring moment 

 created in the torsion wire offsets the moment resulting from the viscous 

 drag of liquid. The viscosity of the liquid is readily calculated from measure- 

 ment of the angle of twist of the suspension wire and certain apparatus 

 constants. As with the capillary viscometer, relative viscosities are readily 

 calculated without knowledge of the dimensions of the instrument. Different 

 devices are available for measuring the twist of the wire. Theoretical con- 

 sideration of the flow pattern of the liquid between the two cylinders shows 

 that the velocity gradient is essentially constant if the gap between the 

 cylinders is small. Most important, extremely low shear gradients can be 

 achieved with ease whereas corresponding values in a capillary viscometer 

 are virtually imattainable. It is regrettable that the Couette viscometer is 

 so intricate, and that most designs do not permit sufficient accuracy of 

 measurements to warrant the widespread adoption of this instrument as 

 the one of choice for viscometry. Kecently (Frei et al., 1957), important 

 modifications in the design and construction have been introduced which 

 change this picture radically, and it is likely that this modified instrument 

 will find wider apphcation in the future. 



Viscosity measurements have also been made for many years by the 

 application of Stokes' law, which describes the rate of fall of a spherical 

 particle through a viscous medium. From knowledge of the densities of the 

 particle and the solution, the radius of the sphere and the measured rate of 

 sedimentation, the viscosity of the solution can be calculated directly. As in 

 the other methods, knowledge of the exact size of the sphere is unnecessary 

 if relative viscosities are desired. Only the relative times for the spherical 

 particle to fall a fixed distance and the densities of the sphere and the two 

 liquids are needed. This method has not been applied widely although the 

 recent widespread production of minute glass and plastic spheres may make 

 this technique more popular. Usually the ball is allowed to fall under the 

 influence of gravity, but some applications have been reported wherein a 

 centrifugal field was employed as a driving force for submicroscopic particles 

 (Schachman and Harrington, 1952). 



d. Interpretation of Viscosity Data. Most experimental data can be ex- 

 pressed in the form of a power series relating the relative viscosity, 7;^.^ j or 

 tj/tjo, as a function of concentration, 



^re, = Vho = 1 + ^C + 5C2 + . . . (4) 



In this equation 17 is the viscosity of the solution and rjo is the viscosity of 

 the solvent. The term, r^^p, the specific viscosity, is used frequently and is 

 defined as {rjlrjo — 1). Equation (4) can be rearranged to give 



r^Jc = A + Bc+. . . (5) 



