
Viscosity of Pitch-like Substances. d01 
agreement with the inverse-fourth-power law. Asan example, 
the values obtained with two cylinders of the same kind of 
pitch and determined at the same temperature are given 
here:— 
R. T. l. U. a 
(em.).  (dyne cm.). (cm.). (steady value). 
a6 09655 10%) 21°) CP SOeI0- + © 9-9 x 10" 
Pet 04% 0 © 4-5 eee hr x 10" 
Fig. 4. 
Bera iae. 
/ 
aS eee Se 
, | 
Re ane 
| 
er | 
lel 
100 
\ 


> : 
Ls 
> 
= 
10 Ie 16 18 20 
ee ie Bae per Min. 
Curve showing relation between Weight and Viscosity. 
The Rate of Dissipation of Strain Energy.—The apparatus 
enables us to show that the strain energy, which we have seen 
to exist in a viscous substance when flowing, will disappear in 
course of time without there being any movement of the 
substance as a whole. 
The rate of dissipation of the strain energy, which becomes 
stored in the cylinder of pitch when rotating, was determined 
by the simple device of gradually removing the w eights, pro- 
ducing torque, at such arate as just to prey ent rotation taking 
place in either direction. 
If the weights are wholly removed, the cylinder turns 
backwards : if left on it continues in its forward rotation. 
Some intermediate weight will just suffice to hold it at 
rest at any moment. This weight must be continuously 
diminished, and finally reaches zero. To admit of this gradual 
removal, the weight of course must not be in one piece 
but must be made up of a large number of small pieces. The 
rate at which the weight must be diminished can afford a 
measure of the rate at which the strain energy in the substance 
2B2 
