In the early 1400s came the invention of the coiled spring or fusee which maintained constant force to the gear wheels of the timekeeper despite the changing tension of its mainspring.

When we take a spring, something that - we are so familiar with now, and the spring has length l l in a relaxed state, k spring constant k, I can extend the spring with some force that I apply.

Constant force spring hanger is a device which puts invariable tension on the thing it hangs in limited displacement.