How a Thermostat Works

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How a Thermostat Works

• Heat and Temperature Unit
• Grade 7 Science

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A thermostat is basically made of three
components1. A Bimetallic Strip2. A
Mercury Switch3. Wires that connect to a
furnace.
Mercury Switch
Bimetallic Strip
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Bimetallic Strip

• A thermometer coil with bimetallic strips
  attached.

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Bimetallic Strip Continued

• A bimetallic strip is a piece of metal made by
  laminating two different types of metal together.
  The metals that make up the strip expand and
  contract when they are heated or cooled. When
  this coiled strip is heated, the metal on the
  inside of the coil expands more and the strip
  tends to unwind.

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Thermal Expansion

• Which metal (A or B) expands fastest?
• Which metal (A or B) contracts fastest?

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When the bimetallic strip is heated, the metal
expands and tilts the mercury switch.
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The mercury conducts electricity. When cold, the
mercury switch is tilted so that it allows
electricity from the wires though and turns on
the furnace. When the room gets warm, the
bimetallic strip expands and tilts the mercury
the opposite direction, shutting the furnace off.