  We report on a direct quantitative comparison between Thom's general
catastrophe theory for systems presenting discontinuous behavior and
experimental reality. It is demonstrated that the model provides a striking
quantitative description of the measured experimental features of the complex
nonlinear system generating the most appealing class of sensors and devices
nowadays used in experiments, namely the Superconducting Quantum Interference
Devices (SQUIDs). The parameter space of the SQUID system that we investigate
displays all the features associated with a butterfly catastrophe, namely a
catastrophe expected for a system having four control parameters and one state
variable.
