  A nonlinear system possessing a natural forbidden band gap can transmit
energy of a signal with a frequency in the gap, as recently shown for the
nonlinear chain of coupled pendula [Phys Rev Lett 89 (2002) 134102]. This
process of nonlinear supratransmission, occurring at a threshold exactly
predictable in many cases, is shown to have a simple experimental realization
with a mechanical chain of pendula coupled by a coil spring. It is then
analyzed in more detail by first going to different (non-integrable) systems
which do sustain nonlinear supratransmission. Then a Josephson transmission
line (one dimensional array of short Josephson junctions coupled through
super-conducting wires), is shown to sustain also nonlinear supratransmission,
though being related to a different class of boundary conditions, and despite
the presence of damping, finiteness and discreteness. Finally the mechanism at
the origin of nonlinear supratransmission is a nonlinear instability, and it is
briefly discussed here.
