There are several types of headphone: crystal and electromagnetic. The electromagnetic type is the most commonly used. They functioning in the same way as speakers, with obvious differences in construction, since they are intended for much lower power. Their main characteristic is their resistance (impedance), from a few ohms to a few thousand ohms. The cross section of an electromagnetic headphone is shown in 8.5. It consists of a horseshoe magnet with poles that hold two coils. These are connected in series. The diaphragm is a thin steel plate. When current flows through the coils, the diaphragm is pulled towards the coils. This moves the air and the result is heard as a faithful reproduction.