Metal detectors are electronic machines that can be used to detect the presence of metal. They can penetrate wood, soil and various nonmetallic items. Electromagnetism is the main principle behind the working of metal detectors. The main components of a metal detector are battery case, receiver antenna, electronic box and a transmitter. A magnetic field is created by the transmitter using the power from the battery. This field magnetizes any metal in its presence. The receiver obtains the electromagnetic signature of the metal substance and transmits a signal to the electronic box. Usually, a speaker is used to increase the strength of the signal. The presence of metal is indicated by a beep sound. There are several types of metal detectors available in the market today. Some common types are walk through detectors, coin detectors and underwater detectors. Metal detectors for hunting relics and those that detect the presence of gold are also quite popular.