232 H. K. SCHACHMAN AND R. C. WILLIAMS 



same. The instrument is composed of a long, narrow glass capillary, of uni- 

 form bore, whicli connects a drainage bulb to a large reservoir chamber, A 

 determination with such an instrument consists of a measurement of the 

 time required for liquid to flow from the drainage bulb through the capillary 

 into the reservoir. Usually the flow is maintained by the hydrostatic head of 

 liquid since, in the construction of the apparatus, the drainage bulb is 

 placed above the reservoir. Through the use of the Poiseuille equation, the 

 viscosity of the liquid can be calculated directly from the measured volume 

 rate of flow, the pressure head, and the length and diameter of the capillary. 

 Such a determination, of course, requires precise knowledge of the various 

 parameters of the instrument. Alternatively, the solution of interest is 

 compared with a reference liquid such as water, and the absolute viscosity of 

 the solution is obtained without detailed knowledge of the dimensions. 

 Only the relative outflow times and relative densities are required if a 

 standard liquid is employed. There are many precautions to be exercised 

 in the design, construction, and use of a viscometer of this type and these 

 have been the subject of detailed investigations. Only one factor remains as 

 a deterrent to acceptance of this tyjie of instrument for all substances. This 

 is an inevitable consequence of the nature of flow through a narrow tube. 

 The rate of movement of the liquid in the different streamlines varies across 

 the tube in a parabolic manner with the maximum rate at the center and 

 zero at the wall. Thus the shear gradient, defined as the rate of change of 

 velocity of the layers of the flowing liquid with respect to the distance 

 separating the layers, varies from zero at the center to some maximum 

 value at the wall. It is this shear gradient (or velocity gradient) which is 

 responsible for the orienting force on the anisometric particles; a precise 

 treatment of the consequent orientation is difficult because of the wide 

 variation in shear gradient across the tube. For spherical particles this 

 limitation is trivial because all orientations of a spherical particle are equiva- 

 lent. However, rodlike particles may be oriented to different extents depend- 

 ing on their exact location relative to the distance from the center of the 

 tube. It should be noted that sufficiently low shear gradients can be achieved 

 in capillary viscometers so that even the anisometric tobacco mosaic virus 

 particles are not oriented j)referentially during flow. Deoxyribonucleic acid, 

 however, is oriented and special precautions are needed so as to allow 

 extrapolation of the experimental results to zero shear gradient. 



Although another type of instrument, the Couette viscometer, meets 

 satisfactorily this one limitation of the caj)illary viscometer, it suffers from 

 other defects. In this instrument the liquid is placed between two concentric 

 cylinders, one of which is caused to turn by an externally applied torque 

 and the other is held suspended from a thin torsion wire. The liquid which is 

 set in motion by the rotation of the outer cylinder imparts a torque on the 



