Fig. 14.—Weight and Extension.
B, rubber 3/16 in. thick; C, 2/16 in. thick; D, 1/16 in. thick. A, theoretical line if extension were proportional to weight.
In this experiment we have been able to stretch (distort) a piece of rubber to more than three times its original length, and afterwards it finally returns to almost its original length: not only so, a piece of rubber cord can be stretched to eight or nine times its original length without fracture. Herein lies its supreme advantage over steel or other springs. Weight for weight more energy can be got or more work be done by stretched (or twisted, or, to speak more correctly, by stretched-twisted) rubber cord than from any form of steel spring.[12] It is true it is stretched—twisted—far beyond what is called the "elastic limit," and its efficiency falls off, but with care not nearly so quickly as is commonly supposed, but in spite of this and other drawbacks its advantages far more than counterbalance these.
§ 3. Experimenting with cords of varying thickness we find that: the extension is inversely proportional to the thickness. If we leave a weight hanging on a piece of rubber cord (stretched, of course, beyond its "elastic limit") we find that: the cord continues to elongate as long as the weight is left on. For example: a 1 lb. weight hung on a piece of rubber cord, 8-1/8 inches long and 1/8 of an inch thick, stretched it—at first—6¼ inches; after two minutes this had increased to 6-5/8 (3/8 of an inch more). One hour later 1/8 of an inch more, and sixteen hours later 1/8 of an inch more, i.e. a sixteen hours' hang produced an additional extension of ¾ of an inch. On a thinner cord (half the thickness) same weight produced an additional extension (after 14 hours) of 10-3/8 in.
N.B.—An elastic cord or spring balance should never have a weight left permanently on it—or be subjected to a distorting force for a longer time than necessary, or it will take a "permanent set," and not return to even approximately its original length or form.
In a rubber cord the extension is directly proportional to the length as well as inversely proportional to the thickness and to the weight suspended—true only within the limits of elasticity.
Fig. 15.—Extension and Increase in Volume.
§ 4. When a Rubber Cord is stretched there is an Increase of Volume.—On stretching a piece of rubber cord to twice its original (natural) length, we should perhaps expect to find that the string would only be half as thick, as would be the case if the volume remained the same. Performing the experiment, and measuring the cord as accurately as possible with a micrometer, measuring to the one-thousandth of an inch, we at once perceive that this is not the case, being about two-thirds of its former volume.
§ 5. In the case of rubber cord used for a motive power on model aeroplanes, the rubber is both twisted and stretched, but chiefly the latter.