A hairdryer, also known as a blow dryer, is an electromechanical device that blows ambient or hot air over damp hair to speed the evaporation of water and allow stronger shaping of the temporary hydrogen bonds in each strand. This allows better styling of the hair, although some forms of drying can leave the strands vulnerable to humidity. The device is powered by an electric motor and a fan that draws air from the outside of the plastic shell, which is shaped to look like a gun.
The heating element is a strip of metal, typically made from nichrome or a nickel-chromium alloy. It looks a bit like a coiled spring, and may be up to 12 in (30 cm) long. The metal expands and bends when heated, and this movement trips a switch that cuts off power to the heating coil.
Modern hair dryers also use a safety mechanism called a bimetallic strip to prevent overheating. The strip is made of sheets of two different metals that expand at different rates, and one is a thinner and weaker material than the other. As the temperature rises, the thin metal sheet expands faster than the thicker metal, and this triggers a thermal fuse that blows to break the circuit.
The electrical system inside the plastic body contains copper wire and switches, and the motor and fan are pre-assembled from components that arrive at the manufacturer pre-assembled. As with other consumer electronics, the hair dryer must pass certain safety tests before it can be sold. This includes a test to make sure it can’t short out and cause a fire, and a special shock protection circuit called a Ground Fault Circuit Interrupter (GFCI) to keep it from electrocuting people if it is dropped into a sink or tub full of water. hairdryer