UNIFICATION OF METHODS IN TESTING MATERIALS. XXIII, 
This method, which is due to Prof. Tetmaier, gives the true 
elongation independently of the length of the test piece. The 
test piece should be divided along its useful length into spaces 4” 
apart to facilitate the measurement of A/, and this has been the 
practice for several years in the Engineering Laboratory of Sydney 
University in all tests in which the author has not been restricted 
by the specification of tests accompanying the test pieces. 
It can be shown however, that the total elongation measured 
in the usual way may be made to give approximately uniform 
results in diverse sections, if the length is suitably chosen thus : 
Let the following proportions be adopted for a useful length of 
8’, viz., 0-8 diameter for circular sections, and 6 : ¢ = 3 for 
rectangular sections. Then to find the length of test piece which 
will give the same value for the total elongation divided by the 
length as the normal section, we assume that the local extensions 
are proportional to the diameters in circular sections, and to the 
ratio of the breadth to the thickness in rectangular sections. 
Experiments on different proportions of test pieces for the same 
material show that this assumption is practically true. Hence 
the following method may be used :— 
Let x be the useful er over which the shootin are 
measured, then— 
x do + Alo = Al, 
_ Al, — Alo 
a Ao 
The general expression for the elongation after rupture is— 
a Ao + Alo = Al 
For a test piece of normal proportions having a length of 8” we 
ave— 
8 Xo + Alo’ = Al, 
wv. Ao + oeek = 
x Hy 
Xo > oa: 
Imposing the condition— 
