352 Prof. Trouton and Mr. Andrews on the 
is dissipated and converted into heat, provided there is a 
linear relation between stress and strain. 
Fig. 5 gives the curve of dissipation obtained from a 
cylinder of pitch. The experiment was not carried to com- 
pletion, and at the end of one hour, on removal of the whole 
Fig. 5. 

50 
20 




WEIGHT. (gmes.) 
| 
0 
800 ~=‘1200 1600 2000 2400 e800 3200 600 
400 
TIME (seconds 5) 
Dissipation Curve for Pitch. 
weight still on the apparatus, the pitch was found to be able 
to recover several degrees. The curve is not a logarithmic 
one as might have been very well expected ; the lower part 
indeed fits well with a rectangular hyperbola. 
This gradual dissipation of strain energy bears an analogy 
to the dissipation of the energy which takes place in an 
electrically strained dielectric which is partly conducting. 
We may look upon the steady state of flow as one in which 
there is a continuous transformation of the elastic strain 
energy into heat, and an equal continuous replenishment of 
the-strain energy from the work done by external torces—a 
kind of dynamic equilibrium. 
This point of view is suggestive as giving an insight into 
processes going on in substances in the viscous flow; namely, 
that the sequence of events is primarily a production of 
elastic strain which is rapidly dissipated by the breaking 
down, so to speak, of the strained material. The fact that 
the strain energy can disappear without any deformation 
occurring, enables us to draw a distinction between plastic 
and viscous substances. In a plastic substance, when held 
under constant deformation beyond its elastic limit, the 
strain energy it then contains does not sensibly lower with 
time; whereas we have seen that the contrary is the case 
with viscous substances. 

