Editing Bistability (section)

== In mechanical systems ==
[[File:Ratchet example.gif|thumbnail|A ratchet in action. Each tooth in the ratchet together with the regions to either side of it constitutes a simple bistable mechanism.]]

Bistability as applied in the design of mechanical systems is more commonly said to be "over centre"—that is, work is done on the system to move it just past the peak, at which point the mechanism goes "over centre" to its secondary stable position. The result is a toggle-type action- work applied to the system below a threshold sufficient to send it 'over center' results in no change to the mechanism's state.

[[spring (device)|Springs]] are a common method of achieving an "over centre" action. A spring attached to a simple two position ratchet-type mechanism can create a button or plunger that is clicked or toggled between two mechanical states. Many [[Ballpoint Pen|ballpoint]] and [[Rollerball Pen|rollerball]] retractable pens employ this type of bistable mechanism.

An even more common example of an over-center device is an ordinary electric wall switch.  These switches are often designed to snap firmly into the "on" or "off" position once the toggle handle has been moved a certain distance past the center-point.

A [[ratchet (device)|ratchet-and-pawl]] is an elaboration—a multi-stable "over center" system used to create irreversible motion.  The pawl goes over center as it is turned in the forward direction. In this case, "over center" refers to the ratchet being stable and "locked" in a given position until clicked forward again; it has nothing to do with the ratchet being unable to turn in the reverse direction.