EFFECT OF TEMPERATURE ON COPPER. 289 
7. Temperature and rate of permanent elongation.—Autographic 
diagrams were taken in tests 2, 3, 8, 9, 10, 11, 41 and 42, and 
these are reproduced in Fig. 6, with the exception of 2 and 3, 
Fig. 6. 
Ordinates actual load in Ibs. : 
Abscisse Extension in inches. 
in which the autographic apparatus failed, and also of 8 in which 
the test was stopped before the piece broke, for the sake of 
obtaining a desired museum specimen. The rate of permanent 
elongation evidently increases rapidly as the temperature rises, 
and the ‘yield point’ shows a corresponding diminution. Since 
the abscissa of the final point of each curve gives the elongation 
for the corresponding test piece, a curve drawn through these 
points would correspond approximately to the curve of elongations 
referred to in § 5. 
8. The elastic limit in tension.—The stress-strain curve for a 
tensile test, No. 31, obtained by means of Marten’s mirror extenso- 
meter, shows clearly the elastic limit as marked by the point of 
departure of the curve from proportionality of stress and strain 
(Fig. 7, curve C.). This limit occurs at about 5,400 Ibs. per 
S—Nov. 3, 1897, 
