Inside a conventional thermostat, a bi-metal coil contracts when cold and expands when warm. The sensor's movement trips a switch to "On" or "Off." The switch may be simply an open contact that closes or it may be a mercury-activated switch. With the latter, mercury in a small, glass vial completes or breaks an electrical circuit when the vial is tilted by the bi-metal coil.
--Don Vandervort
