View Full Version : Hw metal detector works????????

12-14-2006, 10:25 PM
Treasure Hunting
Although scorned by most archaeologists, a metal detector can be fun for those with a casual interest in locating the odd coin or two or perhaps something more significant. Metal detectors use the principles of electromagnetism (first discovered by Joseph Henry in 1831 and first published by Michael Faraday in the same year) in order to detect both magnetised and unmagnetised metallic objects.

So how does it work? Central to all metal detectors is a metallic 'search' coil, sometimes called a search 'loop' which 'sniffs' out metallic objects. An electric alternating current is passed from the metal detector's battery through the search coil. This current induces an alternating electromagnetic field around the search coil. This field is uniform and looks similar to the field which iron filings pick out around a bar magnet.
However, the field is easily distorted by a metallic object, for example a coin or a gold nugget, or an object which is magnetised, such as a nail or naturally magnetic mineral like magnetite. A conducting metallic object (coin) produces an electromagnetic field which oppose the induced electromagnetic field of the search coil. The opposing electromagnetic fields are produced by eddy currents within these objects. When the uniform electromagnetic field of the search coil passes over a metallic object with an eddy current, the detector's field is weakened and distorted. The opposite happens with a magnetic object: in an alternating electromagnetic field (the field produced by our metal detector) the object causes the field to be strengthened and distorted.
So that's how the detector works, but how does the metal detector alert you to your potential treasure? All metal detectors have an audible note produced by the mixing of two oscillators. One oscillator produces a 'reference' frequency which is very similar to that produced by another oscillator circuit within the search coil. The search coil frequency is dependent on the distortion of the electromagnetic field it produces, ie the materials it passes over buried in the ground.

So when a search loop is passed over a conducting metallic object, the detector's electromagnetic field is weakened – this raises the frequency within the search coil oscillator, making an audible difference. When a magnetic material enters the field of the detector and concentrates it, a lower frequency is produced in the search coil oscillator, resulting in an opposite audible beat. Bingo! you can now distinguish between metallic and magnetic objects.

Four principle types of metal detectors are commonly found on the market today – many are very complex, and are more sensitive regarding detection.Metal detectors, Treasure and the Law
If you wish to use a metal detector on land which is not your own, you must get permission from the landowner (not a tenant) before you do so. It is a criminal offence to use a metal detector on land without consent, and you may be sued for damages. Archaeological sites protected by law are called Scheduled Ancient Monuments. Again, it is a criminal offence to use a metal detector on scheduled sites without written agreement from the Secretary of State. You can find out about these restrictions by contacting English Heritage, or Cadw (Wales). Note that Scotland has separate laws regarding the use of metal detectors.

Objects and coins containing precious metals that are over 300 years old, and any metallic prehistoric objects, may be classed as Treasure. Should you find any such artefacts you must report it to the local Coroner. An inquest will decide if the find is Treasure. If so, and you have acted lawfully, you will receive an ex gratia reward if any items are held by a museum.

10-02-2008, 09:44 AM
Cool Info. Duke can you shift this topic to educational materials or can any MOD can do this???

10-22-2008, 02:39 PM
nice post darani...

10-23-2008, 05:06 AM
hmm... gr8 info

10-23-2008, 09:47 AM
hey zara ve u read it?

10-23-2008, 10:17 AM
haan yaar...

10-23-2008, 10:27 AM