The old system

From 1974 to 2007 I have been using the Honeywell Mercury filled switch pictured above. On the left is the (already in 1994 hacked) thermostat with the lid on. The hole is an optical indicator. If you see the mercury drop, the heater should be on. If it isn't, the furnace is on strike. And it needs some convincing (with a monkey wrench) to get going again.
On the right, you see the guts of the thermostat. Under the white disk is the actual temperature sensor (a bimetal spring that expands due to temperature). If the temperature goes below the limit, the glass vessel tips to the left, the mercury drop rolls to the left as well and it shorts two pieces of iron inside the tube, thereby closing the switch. And telling the furnace it's time for some action again.

This marvel of mechanical engineering has some kind of feed-forward coupling as well. When the switch closes, current is fed through a low-Ohm resistor close to the temperature spring. It preheats the sensor so that it will cope for the overshoot due to the water content of the system. The 'finger' on the white disk is used to set the amount of preheating. In a running system, NEVER change the settings of the finger. You will regret it otherwise.

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