1911-12.] 
359 
The Elastic Strength of Flat Plates. 
Plate E. Load 20 lbs. per sq. in. 
At centre, theoretical value of deflection = ‘0197 in. 
„ experimental „ „ = ‘026 — -006 = '020 in. 
Plate F. Load 20 lbs. per sq. in. 
At centre, theoretical value of deflection = ’0061 in. 
„ experimental „ „ = '008 „ 
Most of these results give a practical deflection almost identical with 
the theoretical. There were, however, a few cases in which there was a 
difficulty in determining accurately the intercept, owing to the shape of 
the curve near the origin. Just as to why some cases should show 
exceptions was at first very puzzling, for sometimes types of bad zero error 
were encountered. 
Note on Zero Errors and the General Shape of the 
Deflection-Pressure Curve. 
It was found that on every plate there was a zero error, and after much 
trial and experiment this was found to be due to two causes : — (a) A certain 
initial irregularity of the plate analogous to slight kinks in a wire. A 
small applied pressure took this out. 
( b ) A minute yielding of the fixed edges. At first sight it would seem 
impossible with the apparatus used that there could be the slightest give, 
especially when such pressure could be put upon the studs that the plate 
could be actually compressed at the edges. Yet it was often found that 
there was a minute slackness. With the larger size plates used this slack- 
ness was in general taken out by a few pounds pressure. 
Diagram 4 has been drawn to indicate the nature of the zero error, and 
the general shape of the curve for the large-size plates. There are three 
main portions : — 
1. From O to A. This is curved, and shows the effect of instability of 
the plate being removed. 
2. From A to B. This is a perceptibly straight line and is the true 
elastic line. It usually commences a short distance from the origin and 
continues for a distance that depends upon the point at which permanent 
set begins for the plate. It does not cease at the permanent set point, but 
continues perceptibly straight up to a pressure from two to six times that 
which first produces set. This straight line is always present, is often very 
long, and is the line that must be taken to test the theory. When it is 
produced back on the vertical ordinate, it gives an intercept OS, and the 
value of this intercept must be subtracted from all readings on the straight- 
