then pulled by the direct action of a hydraulic cylinder, the pull being counterbalanced and measured by a long steelyard on which there is a movable weight. After a certain time the piece begins to stretch, and the first sign of this is measured on the piece by a pair of compasses which have been set to a definite and specified length varying from 2 to 10 ins. according to the test specimen. This length is marked on the specimen before the test is begun. The point at which the first signs of permanent stretch are observed is termed the yield point, and the stretching denotes that the limit of perfect elasticity of the material has been passed. The position of the weight on the steelyard at this moment is recorded, after which the pulling is continued until the specimen breaks, the position of the weight being then noted again. The steelyard is marked directly in tons, so that the position of the weight shows without further calculation the total number of tons pull on the specimen. The number of tons divided by the known original area of the section of the specimen in square inches, gives the yield point or the ultimate breaking strength of the material in tons per square inch.
Just before the specimen breaks it pulls out somewhat after the manner of a piece of toffee. The extension of length over that of the specimen, as it was originally, and also the contraction of area where it has broken, are also recorded, and give a measure of the ductility of the material.
