UNIFICATION OF METHODS IN TESTING MATERIALS. XV. 
between the reference points, and the result is only approximate, 
as the elongations are produced, during the course of the test in 
an irregular manner along the length of the test piece, and are 
probably not necessarily uniform at the ruptured section, even 
when that is in the middle of the test piece. The resolutions of 
the Conventions recommend the rejection of all test pieces which 
break outside the middle third of the distance between the refer- 
ence points, which appears desirable in accurate testing unless 
Some such expedient as the foregoing is adopted. 
Dimensions of test pieces.—The Conventions agree that the test 
length should be proportional to the square root of the cross- 
Section, but the American Society adopt for circular sections an 
area of test piece of 600 square millimeters for a useful length of 
200 millimeters, which gives a diameter of 27-64 millimeters, 
whereas, the European standards give lengths respectively of 100, 
150, 200, and 250 millimeters for diameters of 10, 15, 20, and 25 
millimeters, These standards are recommended for all metals. 
In English measure these proportions become lengths of 4”, 6”, 8” 
and 10” for diameters 0-4”, 0:6”, 0-8” and 1:0” respectively, 7.e., 
the length should be ten times the diameter. It is also provided 
in order that the proximity of the heads shall not interfere with 
the observed results, especially in the measurement of elongation, 
the actual length of the cylindrical portion of the test piece should 
be increased by at least 10 mm., it was formerly made 20 mm, 
In testing plates or pieces of square or rectangular cross section, 
the law of similarity of form should be applied as far as possible, 
but with due regard to convenience of preparation. It is there- 
fore desirable to make the width constant in each series, and to 
vary the length and thickness thus :—For plates between }” and 
1’ in thickness, the width should be 1-2’, over 1” in thickness, the 
thickness should be considered as breadth, and the width made 
08". The useful length or that over which the elongations are 
Measured should be 8” for a standard section of 1°2” x 0-4”. 
For testing thin plates under }” in thickness, such as are now 
frequently used for water pipes, the law of similarity probably 
