Constant force springs are a special variety of extension spring. They consist of a spiral of strip material with built-incurvature. When the strip is extended the inherent stress resists the loading force, just as in a common extension spring, but at a nearly constant rate.
In use, the constant force spring is usually mounted with a drum and the free end attached to the loading force. This relationship can be reversed, however, with the free end mounted stationary and the constant force spring itself providing the working force, as with carbon brushes in electrical apparatus.