Household electric heaters; safe use; the thermal relay
The main part of an electric heater is a high-resistance wire or ribbon that heats when current passes. It is made of special alloys like nichrome, fechral or constantan, because these have high resistivity and neither melt nor oxidise at high temperature. Heating elements are open (the coil in air or in an insulator groove), closed non-sealed (for example with ceramic beads) and sealed (the tubular electric heater, TEN). In a TEN the coil sits inside a metal tube in quartz sand or magnesium oxide powder; its life can reach 10 000 hours. Infrared heaters pass heat by radiation, while a microwave oven heats by making the water molecules in food vibrate, and metal dishes are not used in it. A thermal relay contains two metal strips joined side by side (a bimetal): when heated it bends toward the metal with the smaller coefficient of linear expansion and opens a normally closed contact. This is how an iron, kettle or heater is kept from overheating. Safety: never leave a heater unattended or cover it, keep it away from flammables and water, do not touch it with wet hands, and pull the plug out by the plug itself, not the cord. Do not repair a faulty cord or plug yourself: a current of 0.1 A can be fatal. Electrical work and repair are done only by adults and qualified masters.
Diagram game: the teacher shows pictures of an iron, kettle, microwave and heater; groups identify the heating type of each (open, TEN, microwave…) and write three safety rules. Real appliances are not plugged in; a bimetal sample is only shown.