Heating element
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A heating element converts electricity into heat through the process of Joule heating. Electrical current running through the element encounters resistance, resulting in heating of the element.
Most heating elements use Nichrome wire or ribbon as the conductor. Nichrome is an ideal material as it is inexpensive, has relatively high resistance, and does not break down or oxidize in air in its useful temperature range.
There are four kinds of commercial heating elements:
- Calrod (sealed element): a fine coil of Nichrome wire in a ceramic binder, sealed inside a tough metal shell. These can be a straight rod (as in toaster ovens) or curved to fit in a smaller space (such as in electric stoves, ovens, and coffee makers).
- Heat lamp: a tubular high power incandescent lamp usually run at less than maximum power to radiate mostly infrared instead of visible light. These are usually found in radiant space heaters and food warmers.
- PTC ceramic: This material is named for its Positive Thermal Coefficient of resistivity. Most ceramics have a negative coefficient; most metals, a positive one. While metals do become slightly more resistive at higher temperatures, this class of ceramics (often barium titanate and lead titanate composites) has a highly nonlinear thermal response, so that it becomes extremely resistive above a composition-dependent threshold temperature. This behavior causes the material to act as its own thermostat, since current passes when it is cool, and does not when it is hot. Thin films of this material are used in automotive rear-window defrost heaters, and honeycomb-shaped elements are used in more expensive hairdryers and space heaters.
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