OF NITRO-CELLULOSE AND THE LAW OF ITS OPTICAL BEHAVIOUR, 
141 
Six sets of observations are shown in fig. 1 , the loads being applied in 20-lb. 
increments. 
The maximum loads vary from 160 lbs. to 200 lbs., and after the strains reach their 
final value the load is again taken to its initial value and the scale read, to note the 
“semi-permanent set.” The time interval between successive loads varies from one to 
two minutes. It is found that the stress-strain readings so obtained are approximately 
linear, except at the highest values, but in order to obtain as correct a value of 
Young’s Modulus, E, as possible only measurements between 40 and 140 lbs. load are 
used, as the readings between these points are considered to be the most reliable. 
The strain corresponding to this difference of stress of 1256 lbs. per sq. inch is 
0'00354 inch giving a value of E = 355,000. 
x 2 
