Wikijunior:How Things Work/Metal detector
Who invented it? 
In the late 1800s, many scientists and engineers used their knowledge of electricity to try to invent a machine which would find metal. It could be used to find valuable stones, so it could make a lot of money. The German physicist Heinrich Wilhelm Dove invented the basics for the metal detector, which was made into into a metal detector a hundred years later. Early machines were sloppy and used a lot of battery power, and worked for a very short distance. Better versions of the metal detector started to be created in the 1930s.
A scientist named Gerhard Fischer wanted to find a way to know where you are and what direction you are going. It is similar to today's GPS (Global Positioning System). It worked really well, but Dr. Fischer noticed that there were problems in places where the ground had rocks with ore in them (ore is rock that contains metal). He found out that this could be used to find metal. In 1937, he got the first patent for a metal detector. His designs were used as mine detectors during the Second World War (mines are bombs that are hidden underground). The metal detectors were heavy, had vacuum tubes, and needed separate battery packs. After the war, there were plenty of leftover mine detectors on the market. They were bought up by treasure hunters who used them for fun and for gaining money. The hobby of metal detecting had been born.
How does it get power? 
Usually by battery.
How does it work? 
The transmitter coil and the receiver coil.
What does it do? 
It can be used to search for valuable stones and artefacts. It can also be used to find mines and to use as security systems. For example, if someone is carrying a metal gun or bomb a metal detector can find it (although if the bomb is sensitive to magnetism it could cause the bomb to explode instead.) Some kinds of metal detectors can tell what kind of metal is being found and how far away it is.
How does it vary? 
Metal detectors use one of three technologies:
Very low frequency (VLF) Pulse induction (PI) Beat-frequency oscillation (BFO)
How has it changed the world? 
In countries where there have been wars, there are still many mines buried in the ground which continue to injure people. In these regions, metal detectors are the best way to solve the problem and make the areas safe again.
What idea(s) and/or inventions had to be developed before it could be created? 
Fun Facts 
-The Scottish physicist, Alexander Graham Bell, the inventor of the telephone, used a metal detector to try to find a bullet that was in the back of American President James Garfield in 1881.