118 
PROF. W. E. DAL BY: RESEARCHES ON THE ELASTIC 
jV of a second. This time calibration is obtained by placing an interrupter in the lamp 
circuit. Referring to the diagram it will be seen that the yield load was reached in 
about ^ seconds. 
In my 1913 paper I included a diagram taken with an instrument which multiplied 
the extension of the gauge length 150 times so that the elastic part of the curve 
appeared on a scale which enabled its shape to be studied and which enabled the limit 
of proportionality to be identified when such a limit existed. 
In my method of taking these diagrams the test piece is stretched without pause in 
the loading and the spot of light follows without break of continuity every phase of 
the relation between load and extension. Sudden slips of the crystals are duly 
recorded. 
In the usual method the load is applied in steps, pausing at each step to observe 
the extension, so that the piece gets a rest under steady load during the time 
occupied in making the observation of extension. The load-extension curve is thus 
defined by a definite number of points only and peculiarities of form between these 
points are missed. 
I have from time to time continued these elastic researches, and the following paper 
records some of the results obtained with what I call the Optical Recorder of Load 
and Elastic Extension. 
% 
§ 2 . The Test Piece. 
In these researches the gauge length is defined by flanges turned on the test piece 
itself. The ends of the arms of the extensometer rest on these flanges. The 
dimensions of the standard form of test piece used in these researches are shown in 
fig. 2. A shorter gauge length was used for the more ductile metals, but all 
experiments on the iron and steels were done on a 5-inch gauge length. 
I was lead to adopt this form by the many difficulties encountered when pointed 
screws have to be driven into the test piece to define the gauge length. 
r I hese screws cannot be driven properly into hard material like the alloy steels 
which have to be tested nowadays, and in soft material like copper the primitive 
