Heat Anticipator
Thermostats have a neat device called a heat anticipator. The heat
anticipator shuts off the heater before the air inside the thermostat
actually reaches the set temperature. Often, some parts of the house will
reach the set temperature before the part of the house containing the
thermostat does. The anticipator shuts the heater off a little early to give
the heat time to reach the thermostat.
The anticipator is a ring of resistive wire on the dial.
The loop of wire above is actually a resistor. When the heater is running,
the current that controls the heater travels from the mercury switch,
through the yellow wire to the resistive loop. It travels around the loop
until it gets to the wiper, and from there it travels through the hub of the
anticipator ring and down to the circuit board on the bottom layer of the
thermostat. The farther the wiper is positioned (moving clockwise) from the
yellow wire, the more of the resistive wire the current has to pass through.
Like any resistor, this one generates heat when current passes through it.
The farther around the loop the wiper is placed, the more heat is generated
by the resistor. This heat warms the thermometer coil, causing it to unwind
and tip the mercury switch to the right so that the heater shuts off.
http://home.howstuffworks.com/home-thermostat2.htm
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