Nichrome wire is wound into a tight coil that will relax and lengthen as it heats in use. The alloy resists oxidation, so it can glow red for years without burning through.

The coil is threaded around notched cards of mica or ceramic insulators, which hold it in the air stream without letting it touch metal. Copper leads are crimped on at heavier terminals that stay cool.

Sheet steel is punched and bent into the body and grille, then powder-coated. Many cheaper heaters use a molded plastic case instead, and metal cases hold up better if something inside catches fire.

A small electric motor and blade go behind the element, pulling room air in and pushing it out over the hot coil. Because the fan carries heat away, the whole heater can be smaller without overheating.

A bimetal thermostat, a thermal fuse beside the element and a tip-over switch are wired into the circuit. If the fan stalls, the intake clogs or the heater falls over, power is cut.

The case is screwed shut and each unit is powered up to check heat output, fan speed and the safety cutouts. In the US, the safety standards for portable heaters are UL 1278 and UL 1042.