Models of excitability fall into two broad categories: one with a sharp threshold, the other with a narrow but finite threshold region. In an attempt to clarify the notion, we have studied the application of two models of excitability to experiments on a variant of the Belousov–Zhabotinskii reaction in a well‐stirred reactor. An interrupted separatrix model, which has a sharp threshold, is found to describe our experimental results better than a simple nullcline model, which has a finite threshold region.

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