An important factor in selecting material for the cables of suspension bridges is its true elastic limit.  By this term we mean the percentage of the total strength of the material which it can exert continuously without losing its resilience, i.e., its power to resume its former shape and position when stress is removed.  Now, in the case particularly of steel wire as commonly furnished in spiral coils, the curve put into the wire in the process of manufacture seriously diminishes this available sustaining power.

For it is evident that it would be unsafe to subject these cables at any time to a stress beyond their elastic limit.  If, e.g., a snowstorm or a great crowd of people should load a bridge beyond this limit, when the extra weight was removed the cables could not bring the bridge back to its normal place, and the result would be a permanent flattening and weakening of the arch.

By a process invented and patented by Col.  Paine, the wire in the New York and Brooklyn bridge was furnished straight instead of curved.  Now, if a short piece of common steel wire is taken from the coil, and pulled toward a straight position, and then released, it springs back into its former curve; but if a short piece of the straight-furnished wire that was put into this bridge is bent, and then released, it springs back toward its straight position.

It is easy to see that if a curved wire is pulled straight, there must occur a distention of the particles on the inside of the curve and a compression of those on the outside.  The inside is in fact strained past its elastic limit before any stress comes upon the outside.  Hence, after the wire has been pulled straight, the elastic limit of only a portion of it can be taken into the account in calculating the load that can safely be put upon it.  In the case of curved steel wire pulled straight, its ultimate strength was found to be only about 90 per cent. that of similar wire furnished straight by this process.  The superior ductility of iron wire in some measure compensates for the distention of the particles on the inside of the curve, and that is a reason why it has heretofore been used for suspension bridges.  But with straight steel wire there is no such distention, and its entire elastic limit is available.  This elastic limit is 66 per cent. of the ultimate strength, and, besides, that ultimate strength is 10 per cent. greater than that of similar curved wire.  Thus if we have a curved steel wire large enough to sustain 1,000 lb. without breaking, a similar straight wire, such as those in this bridge, will hold up 1,100 lb., and 66 per cent. of this 1,100 lb = 720 lb.