Springs are useful as machine parts, because of their capacity for yielding to force without permanently losing their shape - technically called their "permanent set." Wound springs possess potential energy, because at some previous time work has been performed upon them in the winding. Coiled springs in watches and clocks which set the mechanism in motion, are an illustration. Steel is superior to all other materials for the manufacture of springs, but must be protected when exposed to dampness; otherwise it will rust.
The force of a spring is not exactly uniform in its action, for it has its greatest energy when most bent or most tightly wound. Since the elastic force of a spring is not affected by the force of gravitation, it is used to ascertain the amount of the earth's attraction (pull or weight) in various places. This is done by the use of a cylindrical spring balance to which a hook or ring is fastened (Fig. 41). The object to be weighed is hung from the hook which pulls the spring in proportion to the weight. From graduations on the scale it is possible to read directly the weight of the commodity.