If a load of 4200 lb. is suspended from a copper rod 100 in. long and 1 sq. in. in cross section, the rod will be stretched slightly until it is 100.02 inches long. The external force (weight of. the load) tending to produce elongation will then be exactly counterbalanced by the elastic force produced in the rod and tending to restore its original size and shape; and the load will remain at rest. The external force and the equal restoring force, in any case of elasticity, are each called a stress; the change of size or shape in the body is called a strain. So long as the elastic limit is not exceeded, the elasticity of any body is measured by the ratio of stress to strain; that is,
In accordance with this expression the elastic limit may be defined as the maximum force that can be applied to a body without producing a permanent change in its volume or form. It is commonly said that rubber is very elastic, because it can be stretched to two or three times its length without exceeding its elastic limit; that is, it has a wide range of perfect elasticity. But as measured in physics, substances like copper, steel, marble, and ivory are far more highly elastic than rubber, because a much greater force is required to produce a given change in them.
In the measurement of elasticity many devices are used. One for testing the torsion, or twist, of a rod is shown in Fig.1.
FIG.1
This consists of some form of stable clamp to which one end of the rod is fastened. The other end of the rod is clamped in the axis of a wheel to the rim of which the twist is applied by weights. The reading pointer may be fastened to the rod at any distance from the fixed clamp and on making the experiment the relation between the weights used (stress) and the twist (strain) produced is determined. The following results of an experiment will serve as an example of the method. The rod was of cypress, 1 cm. square and 100 cm. long. An examination of the curve (Fig. 2) shows that the twist was proportional to the pull up to 1100 g. Beyond that pull it was not. That is, the limit of elasticity was reached at that point.
Demonstration. - Make a paste by rubbing some lampblack into kerosene, and put a thin coating upon a flat slab of iron or stone. Place a large marble upon the slab. Notice how small a part of the marble touches the slab. Drop the marble from a height of 6 or 8 ft., and notice both the height to which rebounds and the increased size of the contact between the marble and the slab. Repeat with a steel ball such as would be used in ball bearings, and with a small rubber ball.
FIG.2
