Telegraph poles under compression. Wires under tension.
Girders.
Here let us for a moment dwell upon the two opposite ways in which strength may be brought into play, as either compression or tension is resisted. An example presenting both is a telegraph pole, with well-balanced burdens of wires. Its own weight and its load of wires, compress it, as we can prove by measuring the pole as stretched upon the ground before being set in place, and then after it is erected and duly laden. Should this downward thrust be excessive, the pole would be crushed and broken down. The strung wires are not in compression, but in the contrary case of tension, and are therefore somewhat lengthened as they pass from one pole to the next. Now observe a mass first subjected to compression, and next to tension. In bearing a pound weight a rubber cylinder is compressed and protrudes; when the weight is suspended from this cylinder, the rubber is lengthened by tension. In each case the effect is vastly greater than with wood or steel, because rubber has so much less stiffness than they have.
Rubber
cylinder.
Flattened by
compression.
Lengthened
by tension.