The basic idea behind hair dryers is pretty simple, but producing one for mass consumption requires some hard thinking about safety features. Manufacturers have to predict how their hair dryer might be misused. They then try to design a product that will be safe in the widest variety of conditions.
Besides the ground fault circuit interrupters we learned about on the first page, here are some other safety features hair dryers commonly have:
- Safety cut-off switch - Your scalp can be burned by temperatures more than 140 degrees Fahrenheit (approximately 60 degrees Celsius) [source: Hardin]. To ensure that the air coming out of the barrel never nears this temperature, hair dryers have some type of heat sensor that trips the circuit and shuts off the motor when the temperature rises too much. This hair dryer and many others rely on a simple bimetallic strip as a cut off switch.
- Bimetallic strip - Made out of sheets of two metals, both expand when heated but at different rates. When the temperature rises inside the hair dryer, the strip heats up and bends because one metal sheet has grown larger than the other. When it reaches a certain point, it trips a switch that cuts off power to the hair dryer. (For more information on bimetallic strips, see How Thermometers Work).
- Thermal fuse - For further protection against overheating and catching fire, there is often a thermal fuse included in the heating element circuit. This fuse will blow and break the circuit if the temperature and current are excessively high.
- Insulation - Without proper insulation, the outside of the hair dryer would become extremely hot to the touch. If you grabbed it by the barrel after using it, it might seriously burn your hand. To prevent this, hair dryers have a heat shield of insulating material that lines the plastic barrel.
- Protective screens - When air is drawn into the hair dryer as the fan blades turn, other things outside the hair dryer are also pulled toward the air intake. This is why you'll find a wire screen covering the air holes on either side of the dryer. After you've used a hair dryer for a while, you'll find a large amount of lint building up on the outside of the screen. If this were to build up inside the hair dryer, it would be scorched by the heating element or might even clog the motor itself.Even with this screen in place, you'll need to periodically pick lint off the screen. Too much lint can block the airflow into the dryer, and the hair dryer will overheat with less air carrying away the heat generated by the nichrome coil or other type of heating element. Newer hair dryers have incorporated some technology from the clothes dryer: a removable lint screen that's easier to clean.
- Front grill - The end of the barrel of a hair dryer is covered by a grill made out of material that can withstand the heat coming from the dryer. This screen makes it difficult for small children (or other especially inquisitive people) to stick their fingers or other objects down the barrel of the dryer, where they could be burned by contact with the heating element.
For more information on other household appliances showing their warm, fuzzy side -- whether by drying your clothes, toasting your bread or starting your car -- get your motor going on the next page with a bunch of interesting links.