How a traditional iron switches its heater on and off
Many traditional electric irons regulate heat with a strip made from two bonded metals. Because the metals expand by different amounts when heated, the strip bends and opens an electrical contact. As it cools, it moves back, allowing the heater to switch on again and repeat the temperature-control cycle.
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Two metals turn temperature into movement
A traditional mechanical thermostat can use a bimetallic strip: two different metals firmly joined together. Their rates of thermal expansion differ.
When the strip heats, one layer tries to lengthen more than the other. Because the layers are bonded, the strip bends. In the iron's control mechanism, that movement operates contacts that interrupt power to the heater.
As the mechanism cools, the strip moves back and the contacts reconnect. The heater cycles rather than staying continuously on at full power.
- Heating bends the strip towards the switching point.
- Opening the contacts stops electrical heating.
- Cooling allows the next heating cycle.
The temperature dial changes the switching arrangement. Irons with electronic controls can use different sensors, so this describes the common mechanical design rather than every model.