OF NITRO-CELLULOSE AND THE LAW OF ITS OPTICAL BEHAVIOUR. 
147 
Now in general there is an initial retardation which is independent of any load. 
Let this correspond to stresses F, F 0 . Then the condition Of = Of becomes 
C(/+F) = C 0 (/ 0 + F 0 ) 
0 . df = C 0 . df, 
= reciprocal of slope of the stress/equivalent stress, 
M 
which affords a convenient relation for examining the experimental data. 
Turning now to the further experimental data upon the stress-strain properties of 
nitro-cellulose in tension a number of experiments have been made upon material of 
varying age and thickness, and these are plotted in tire accompanying fig. 5, to show 
their characteristic properties under loads which sometimes exceed very considerably 
the elastic limits of the material. 
With the thinnest specimens y 1 *- inch thick it was not found possible to obtain a 
reliable value of Poisson’s ratio, but Young’s modulus, E, has been found, and the 
measurements plotted in fig. 5 show a characteristic feature that, although the first 
test is carried well beyond the elastic range, as soon as the load is removed a total 
extension of 0*0608 inch is reduced to 0'0070 inch or only 0*001 inch more than 
obtained at the commencement of this test. Moreover the value of the modulus 
changes less than 2 per cent, under these circumstances due to the earlier loading. 
It is also large as the skin effect is pronounced. These general characteristics are 
also observable in the measurements recorded in this figure for much thicker 
material if due allowance is made for the diminished effect of the surface layers. The 
capacity of returning to its original shape after high loads is still more marked in the 
next series of experiments on material ^ inch thick, fig. 6, in which a stress of nearly 
5000 lbs. per sq. inch is reached in the first experiment (curve l) with nearly complete 
recovery, and when further loads with maxima varying from 4000 to 5000 lbs. per 
sq. inch are applied (curves 2 to 8) these give almost identical values of Young’s 
modulus on the straight part of the curve until the ninth loading, where there is a 
sudden fall to E = 261,000 with an extension of 0*1045 inch corresponding to the 
initial load of 20 lbs. After this, with the considerable initial extension of 0*4290 
inch, there is a great rise in the modulus. The value of Poisson’s ratio is very 
constant and is here found to reach the highest value of m = - = 2*7. 
<J 
Succeeding experiments on still thicker material confirm these results, and with 
the exception of the f-inch plate, the load extension curves agree in their linear 
character up to about 2000 lbs. per sq. inch, although the specimens differ in age and 
possibly also in composition. They have, however, the common feature of possessing 
VOL. CCXXI.-A. 
or differentiating, 
therefore 
C 
a 
df ; = 
df 
Y 
