526 hersey: viscosimeters 



design the instrument so that it shall have any desired character- 

 istics ; referring to experiment only after the instrument has been 

 set up ready for use, and then only to get a more accurate value 

 of the single calibration constant which is required, namely r , 

 the deflection at unit speed in water. 



The constants of performance of such an instrument, besides 

 t , are: the free period t; the stress in the suspension at unit deflec- 

 tion; the lag, or time required to attain a stated fraction of the 

 final deflection; and the viscosity for critical damping. Of these 

 to and t are the most important, and are given by 



r = 256ir£-° — ^?- (2) 



and 



i = htez in (3) 



d 2 " rj 



in which mo is the viscosity of the standardizing liquid, rj the 

 shear modulus of the suspension (assumed circular; for a ribbon 

 the coefficient 256 t would give way to some larger number) ; 

 I the length and d the diameter of the suspension, H the height 

 of the inner cylinder, R the ratio of outer to inner radii, and I 

 the moment of inertia of the suspended system. Note from (2) 

 the insensitiveness of the instrument to changes in outer radius 

 when the clearance is large. It is desirable that t be large, 

 and imperative that t be small. 



Construction and use. A simple form of this instrument, 

 roughly constructed for immediate use, had the calculated values 

 t = 0.32 radian /r. p. s. in water at 20°C, and t = 6 sec; and 

 the observed values t = 0.38 and t = 5. Deflections were read 

 off by a pointer and graduated circle, and, subject to deviations 

 of several per cent, were found proportional to the speed, and 

 therefore to the viscosity. An ordinary test tube forms the 

 outer cylinder. In changing samples, one test tube is bodily 

 removed, and the next inserted. Thus 30 cc. is a sufficiently 

 large sample, and the container need not be cleaned. This use 

 of the test tube, together with the fact that the shape of the 



