inside the tubing such as a membrane or miniature piston etc. to harden or relax the spring, much like it happens with water pressure inside a garden hose. Alternatively tubing's cross-section is chosen of a shape that it changes its area when tubing is subjected to torsional deformation: change of the cross-section area translates into change of tubing's inside volume and the flow of oil in/out of the spring that can be controlled by valve thereby controlling stiffness. There are many other designs of springs of hollow tubing which can change stiffness with any desired frequency, change stiffness by a multiple or move like a linear actuator in addition to its spring qualities. Arc spring A pre-curved or arc-shaped helical compression spring, which is able to transmit a torque around an axis. Volute spring A compression coil spring in the form of a cone so that under compression the coils are not forced against each other, thus permitting longer travel. Balance spring Also known as a hairspring. A delicate spiral spring used in watches, galvanometers, and places where electricity must be carried to partially rotating devices such as steering wheels without hindering the rotation. Leaf spring A flat spring used in vehicle suspensions, electrical switches, and bows. V-spring Used in antique firearm mechanisms such as the wheellock, flintlock and percussion cap locks. Also door-lock spring, as used in antique door latch mechanisms. Other types Other types include: Belleville washer A disc shaped spring commonly used to apply tension to a bolt (and also in the initiation mechanism of pressure-activated landmines) Constant-force spring A tightly rolled ribbon that exerts a nearly constant force as it is unrolled Gas spring A volume of compressed gas. Ideal Spring A notional spring used in physics: it has no weight, mass, or damping losses. The force exerted by the spring is proportional to the distance the spring is stretched or compressed from its relaxed position. Mainspring A spiral ribbon shaped spring used as a power store of clockwork mechanisms: watches, clocks, music boxes, windup toys, and mechanically powered flashlights Negator spring A thin metal band slightly concave in cross-section. When coiled it adopts a flat cross-section but when unrolled it returns to its former curve, thus producing a constant force throughout the displacement and negating any tendency to re-wind. The most common application is the retracting steel tape rule. Progressive rate coil springs A coil