The helical spring, is the most commonly used mechanical spring in which a wire is wrapped in a coil that resembles a screw thread. It can be designed to carry, pull, or push loads. Twisted helical (torsion) springs are used in engine starters and hinges. The helical spring is suspended vertically from a rigid support. The pointer is attached horizontally to the free end of spring. A meter scale is kept vertically in such a way that the tip of the pointer is over the divisions of the scale; but does not touch the scale. Helical spring works on the principle of Hooke’s Law. Hooke’s Law states that within the limit of elasticity, stress applied is directly proportional to the strain produced.
This video explains how to find the force constant of a helical spring by plotting a graph between load and extension.