244 
of drawing, but they are not so after hardening and drying. The cross- 
section usually becomes quite irregular. This indicates a condition of 
internal strain which acts to lower the breaking strength of the thread. 
Coarse threads would be subject to greater strains than fine threads, and 
therefore we should expect them to show a smaller tensile strength than 
the fine threads. This is in accord with the results of experiment as 
shown by the data of Table I. 
TABLE I. 
Cross Section in Breaking Strength Tensile Strength in 
sq. mm. in grams. Kgm. per sq. em. 
000835 21.6 3,305 
000919 21.5 2,340 
.001002 21.1 2,106 
.001334 ol. 2,778 
.001670 35.0 2,096 
.002610 58.2 25230 
004524 96.5 2,133 
006729 138.1 2,052 
.013920 Die 1,519 
035900 400.1 1,114 
. 180300 800.0 614 
. 264900 2,850.0 1,076 
. 608900 5,600.0 919 
1.709200 6,100.0 opt 
Inasmuch as the error in measuring the breaking strength of any par- 
ticular thread was relatively small and all strains tended to decrease that 
strength as measured, I have included in Table I only maximum readings; 
that is, readings which gave the greatest values of the tensile strength for 
the several sizes of threads. Average readings gave results some twenty 
per cent lower. 
Curve A of Fig. 1 is a plot of some of the individual maximum read- 
ings of Table I. It will be observed that the measured tensile strength in- 
creases very rapidly as the threads are made thinner. <A similar increase 
takes place in wires and glass threads. and is attributed to a “skin effect.” 
This increase is shown in Curve B, Fig. 1, which represents the results ob- 
