Even though hair dryers haven’t been around long, millions of people in modern society today use hair dryers on a daily basis. A hair dryer or blow dryer is an electromechanical device designed to blow normal or hot air over damp hair, in order to accelerate the evaporation of water particles and dry the hair. Hair dryers were invented around the end of the 19th century. The handheld, household hair dryer first appeared in 1920.
Normally, evaporation is controlled by relative humidity — the ratio of the amount of water the air holds to the amount it could hold. For example, if the relative humidity is 90%, that means that the air is holding 90% of its maximum volume of water. When air is heated, its relative humidity decreases. It still has the same amount of water, but it can hold more. The lower the relative humidity, the more easily water evaporates. Therefore, hot air will dry your hair faster, since the water in your hair will evaporate more quickly.
How Does a Hair Dryer Work?
The two important components of a hair dryer are the electric fan and the heating element (Fig. 1). The heating element is made out of nichrome wire, which is a resistor. A resistor resists the flow of electric energy, turning it into heat energy. In a hair drier, the air blows past the resistor, absorbing heat as it passes. Electric hair dryers work by blowing room temperature air in through the vents. The air then passes over nichrome wire coils that heat it, making it able to blow the hair dry with hot air. The process of air blowing on the hair speeds up water evaporation. Applying power to the hair dryer allows the motor to start spinning the fan inside, and that is what draws the air in through small air holes on the side of the dryer.
Fig. 1 Inside a hair dryer: Heating element on the right and motor-driven fan on the left.