The circular part has a bimetallic strip. The two metals in the strip expand and contract to temperature changes at different rates. This causes the end of the spiral to move clockwise or counterclockwise.
As the temperature heats up, the strip moves to the right, allowing it to break the circuit.
As the temperature cools down, the strip moves to the left, closing the circuit.
http://en.wikipedia.org/wiki/Bimetallic_...
First determine that the problem is really in the thermostat. Put a voltmeter across the two terminals coming out of that glass mercury switch. It should measure either 0 volts if the mercury is shorting the terminals, or maybe 24, 36 volts or so if the mercury is not shorting the terminals. Now either turn the knob or somehow tilt the switch so that the mercury is in the other position. You should get the other reading with your voltmeter. And also, the furnace should either turn on or off. If there is a change in the switch voltage, but no change in the furnace, the problem is not in the thermostat.
If, on the other hand, the furnace does turn on and off, there is something wrong with the thermostat. Behind that white dial is a coiled strip of metal. As the air temperature rises and falls, that strip unwinds or winds slightly, just enough to tip the mercury switch one way or the other. Check to see that there isn't anything stuck in there, and that the coiled strip isn't broken. It's not too expensive to get a new thermostat at the hardware store, by the way.
All the mercury does is bridge the contacts so electricity can flow.
No bridge, no electricity, the heater does not turn on.
The mercury switch is a tilt switch.
It does not expand and contract like in a thermometer to turn the heater on or off.
There is a coil of metal that moves that little bulb of mercury. If that coil is "adjusted wrong" then the mercury switch will tilt into position at the wrong temperature and will tilt out of position at the wrong temperature.
I don't know what voltage range the heater supplies to this switch. If the control for the heater is not working, then there is no power for the switch to work with. An electrician will study the heater and make voltage measurements inside the heater, and if all measurements are good in the heater, then the electrician will check the wiring to the thermostat.
I can't help you much past this. Typically the voltage levels on the thermostat switch are small, but they could be as small as 12 volts or as large as 36 volts. They could be DC or they could be AC. If the heater has a thermocouple-type control the voltage will be as small as 3 volts DC. These voltages are relatively safe and easy to work with.
It is rare that thermostats run at 120 volts. Too dangerous in residences.
I think you need repair service so you don't accidentally injure yourself trying to figure this out.
You have a good answer.
Keep in mind that a mercury switch thermostat must
be leveled when installed and remain level in use.
The thermostat will probably have heating and cooling
options. These must be selected appropriately.
The thermostat manufacturer will provide wiring diagrams
online for the wire colors which match the thermostat
terminals. If the thermostat's wiring has been disturbed it
should be checked.
If your thermostat has only two terminals and two wires
(red and white) you should have 22-27VAC between red and white,
and connecting red and white should start the heater.
If not, seek professional attention because the heating system
uses multiple safeties and interlocks which are essentially
unique to the device, and annual service often results in
fuel cost reduction which pays for the service call.
To add a couple comments... The mercury switch will not always be horizontal, in fact it can't be horizontal all the time, the Hg has to run one way to make the connection and the other way to turn off the connection.
Get a hair dryer. If the mercury switch does not tip one way when it is cold and the other way when it is hot, the coiled spring (thermal expansion material) is not working to tilt it. It may just be that one end of the coil broke off from the attachment point.
Can someone explain how this works?
If you turn the knob toward the right(higher temp) the mercury shifts toward the left. How does it distinguish between temps?
I've deduced that if the mercury is on the right, it shuts off(while set on "heat"), but if the thermostat is set anywhere between 75° and max, the mercury is on the left and turns on the heat.
Our heater is faulty and I was going to try to fix it, but I don't know if the vial of mercury is off balance, or something, because I am failing to understand how it would distinguish between different temps.
It seems as though the mercury completes the circuit and kicks the heater on, but an increase in temp would expand the mercury and keep the circuit completed, thus, the heater would stay on forever...I'm confused
