A thermostat's basic function is to regulate the operation
of heating and cooling equipment in your home. You set the thermostat
to the desired temperature and the thermostat turns the equipment on
and off to maintain that temperature. A thermostat is essentially an
automatic switch. The thermostat senses the temperature and turns the
air conditioner or heater on and off as required to maintain the desired
There are two basic types of thermostats: electromechanical
and electronic. They both do the same thing but they do it in different
ways. Additionally, of the two basic types, there are several varieties
from which to choose.
An electromechanical thermostat typically uses a bi-metal
coil or strip which moves as it expands and contracts with temperature
changes. Mounted on the bi-metal strip is a glass vial partially filled
with mercury. As the vial is tilted on the back on the bi-metal strip,
the mercury flows to the end of the vial where two electrical contacts
are exposed. When the mercury envelopes those electrical contacts,
a low voltage circuit is made, thanks to the fact that mercury is conductive.
This works just like a light switch and makes a circuit to turn on
the heater or A/C.
An electronic thermostat operates in much the same way
as an electromechanical model except that instead of the moving bi-metal
strip and mercury switch, it uses a sensor to detect temperature levels.
When the temperature differs from the preselected temperature the thermostat
electronically makes the circuit to the heater or A/C.