346 Day — Residual Viscosity on Thermal Expansion. 



that temperature. When this temperature is reached the 

 residual viscosity again manifests itself, and the rubber under- 

 goes further gradual deformation, which vanishes in its turn. 



Thus, the hard rubber tube was evidently stretched at the 

 relatively high temperature of manufacture. The stress was 

 locked up in the tube and maintained at ordinary temperatures. 

 Between 50° and 64°, however, this was not so, and a viscous 

 flow took place, resulting in a shortening of the tube, as shown 

 by the diagram. 



The second measurement of the expansion shows that, dur- 

 ing the previous exposure to the temperature 50° . . . 64°, excess 

 of strain was nearly wiped out. When the temperature 64° 

 was again reached, however, the viscous yielding reappeared. 



Phenomena in tempered steel analogous to this in rubber 

 have been observed in great detail by Barus and Strouhal,* and 

 they have made use of their observations in the production of 

 steel magnets of remarkably stable qualities. They found that 

 steel possessed residual viscosity, which manifested itself as an 

 irregular change of the magnetization in permanent magnets 

 when exposed to relatively high temperatures. By boiling 

 their magnets in water for many hours, they found that they 

 could remove such residual viscosity and render them viscously 

 stable below 100°. This treatment is applied successively 

 both to remove excess of mechanical strain and of magnetiza- 

 tion. 



A practical result of a knowledge of this quality would be to 

 secure rubber of a stable molecular condition by subjecting it 

 to a heating process ; e. g, by boiling it in water for a con- 

 siderable length of time. 



Although the viscous motion in metals is much smaller, it 

 would, nevertheless, seem advisable to submit rods intended as 

 standards of length to a thorough boiling in water prior to their 

 final calibration, particularly when the metal is drawn or 

 rolled. 



Wilson Physical Laboratory, 

 Brown University, Providence, R. I. 



* Bulletin No. 73, 1891, U. S. Geological Survey. 



