TRANSACTIONS OF SECTION J. 435 



plied gradually, which is balanced while pumping goes on by 

 winding the poise- weight along the steelyard until frpccture 

 occurs. To test a piece of iron or steel in compression, it 

 should be prepared to a uniform size, parallel throughout, and 

 square at the ends. To test a rolled girder, a bull-iron, or a 

 rail, &c., it is merely necessary to support it across the knife- 

 edges of the cross-head, and apply the load with a knife-edge 

 in tlie centre. 



If it is desired to test the tensile strength, the elasticity, 

 and the ductility of a piece of steel or iron, a light lever mul- 

 tiplying 100:1 is attached to the specimen, which is capable 

 of measuring small elongations. This apparatus will be sub- 

 sequently referred to. The load is applied, and the move- 

 ment of the long arm of the lever shows that the specimen 

 stretches, and that up to a certain point, by no means easy 

 to determine exactly, but generally about 11 or 12 tons per 

 square inch of the sectional area of the specimen, the stretch- 

 ing is uniform and regular : thus, with a load of 1 ton per 

 square inch, the specimen will stretch about ^lihoT) P^^'^ o^ its 

 length, which, for a length of lOin. under test, will be xTiW/i^^- 

 With a load of 2 tons per square inch, the elongation will be 

 T^^joi"- o^ 6"o'oii^- '> ^^^^ with 3, 4, 5, 6, 7, &c., tons per square inch, 

 the elongations will be 3, 4, 6, 6, 7 times xiiWi^- I'espectively, 

 and generally the elongations are practically proportional to 

 the loads producing them. In other words, the specimen is 

 said to be perfectly elastic for these loads ; which is shown by 

 releasing the pressure, when the specimen practically springs 

 back to its original length of lOin. But this so-called elasti- 

 city has a limit, which in wrought-iron is generally about 11 

 or 12 tons per square inch, and in mild boiler steel about 

 18 tons per square inch, after which the elongations increase 

 much more rapidly than the loads producing them, and the 

 material behaves as if it were plastic, until the specimen frac- 

 tures, which generally occurs at from 20 to 24 toiis per square 

 inch for wrought-iron, and from 24 to 30 tons for mild boiler 

 steel; the elongations at the point where the material ceases 

 to be "elastic " (the so-called limit of elasticity) being for an 

 area of one square inch about Y-^oin-, whereas the total elonga- 

 tion at fracture will be from lin. to l-sin. for ordinary iron, and 

 2in. to 3-^-in. for steel, on a length of lOin. 



Again, it will be observed that the area of the specimen at 

 the fracture is smaller than the original area. The elongation 

 is generally expressed in terms of the Icngtli of the specimen 

 tested, and the contraction of area in terms of the original 

 area : thus, we say the percentage of elongation in ordinary 

 iron is about 10 per cent. ; its contraction of area is about 12 

 per cent. 



The elongations and contractions so expressed are measures 



