A<unk>  is, in principle, any piece of wire, whose end is inserted, on one side, into a bracket or a tube, or included in the acrylic of a removable appliance,  while the other one is tied to another unit, with only a one-point contact. When using the<unk> , the orthodontist can easily estimate the force system  on both units, just measuring the length of the appliance and its force by means of a dynamometer -in the direction of its activation- and thereby predict the clinical result
Cantilever mechanics 
Molecular sieve
Pressure gradient 
Heat transfer
Oxygen permeability
Dynamical system
Blood flow
Fluid statics