Thermostats use a mercury switch to activate them (well MOST do, i've seen some that don't, but most do)... when the mercury cools down it moves to one side because its heavier or lighter (i forget which) and vice verse when it gets warm.


Heat tape uses a resistance type switch to activate/deactivate them... Heat makes resistance which reduces overall voltage thru the circuit which is how it controls the temperature.
Ok yes, sure, that's what I meant to say..lol
Rusty