From your description it is a tube filled with alcohool or oil and it
expands a diaphragm which moves a hysteresis switch. Unlike the gas the
liquid is incompressible and when it contracts (when cooled) it can pull
the membrane after it even against a strong spring. Conversely it will
push even harder against the membrane when warmed (expanding). A gas can
only produce a small fraction of the force possible with a liquid.

Peter

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