Diagram of string models used for EEG. (a) A string attached at its ends but unattached to local structures; the associated detail shows the forces acting on a small piece between and . Forces acting on each side of this small piece of string are equal to the tension in the string. The curve adopted by the perturbed string is associated with angles and at and respectively. (b) shows a detail of the same string but now there is an external force, a local effect (a local spring), and the string is embedded in a damping medium (molasses).