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Author Topic: Setting off metal detectors
DemonWolf
Ding Dong! Merrily on High Definition TV


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Now from what I understand, a metal detector operats by setting up a magnetic feild and when a ferris metal (made with iron) passes through the feild is disrupted and the alarm goes off.

Following that logic, copper, lead, and other non-ferris metals should pass through undetected. So why does my brass belt buckle set off the metal detector? Why does the bullet in my friends arm set it off? Bullets are made of lead, lead should pass through, right? I've also heard of people detcting gold with hand-held units (treasure hunters, etc.).

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TrekkerScout
Deck the Malls


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Actually, any metal can set off a metal detector. When any metal passes through a magnetic field, an electrical current is generated in that piece of metal. The induced electrical current in turn creates its own magnetic field. It is this second magnetic field that is being detected by the metal detector. It is all based on Faraday's law of induction. Ferris metals just have the characteristic of being able to create larger magnetic fields than non-ferris metals and are therefore easier to detect.
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kmcm
We Three Blings


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I know titanium doesn't set it off. I've been thru many a metal detector and it hasn't set it off at all.

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DemonWolf
Ding Dong! Merrily on High Definition TV


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Thanks TrekkerScout. That makes alot of sense. I had wonder if perhaps it was simply far more common to add iron to other metals than I had previously thought.
For example, lead is pretty soft. Adding iron to the lead would make a stronger alloy, and in the case of a bullet, cause more damage or allow for better penetration.

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shivaskeeper
I'm Dreaming of a White Sale


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quote:
I know titanium doesn't set it off. I've been thru many a metal detector and it hasn't set it off at all.
It depends on how much Ti you have in you. At one point, for about 18 months, I had enough in my hand to set of full body scanners and hand-helds.

Greg

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TrekkerScout
Deck the Malls


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quote:
Originally posted by kmcm:
I know titanium doesn't set it off. I've been thru many a metal detector and it hasn't set it off at all.

Titanium can set off a metal detector if either you have a large enough quantity (as shivaskeeper has stated) or the sensitivity of the detector is increased. Many metal detectors are set at a level that they won't be set off when small quantities of non-ferris metals (such as common jewelry metals like titanium, gold, and silver) pass through. However, I've been through detectors sensitive enough that the zipper in my blue jeans has set off the alarm.
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Goombah
I'll Be Home for After Christmas Sales


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quote:
Originally posted by kmcm:
I know titanium doesn't set it off. I've been thru many a metal detector and it hasn't set it off at all.

I know that my sister's prosthetic knees will set off detectors. To be fair though, I don't honestly know if her implants are made from titanium, stainless or compressed oatmeal.

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Troberg
Angels Wii Have Heard on High


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Any material that is an electric conductor will set it off.

Basically, the detector is an inductive loop that is pulsing at a certain frequency, creating a weak alternating magnetic field. This, in turn, creates eddy currents on the surface of conductive materials (which usually are metals) which changes the inductance of the loop. This change causes the frequency of the loop to change, which can be detected.

This sounds complicated, but the electronics behind it is really simple, basically it is just a resonant feedback loop.

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/Troberg

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Ganzfeld
Let There Be PCs on Earth


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Wee nitpick: Guys, metals with iron are "ferrous". Ferris is that kid who took off school to borrow his dad's car or the giant wheel you ride at the fair.
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A Cat Named Easter
Catfish on a Hot Tin Roof


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Just to nitpick a little bit more, Ferris and his friends borrowed Cameron's dad's car.

A Cat ~only the greatest movie EVER~ Named Easter

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kmcm
We Three Blings


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How odd, I have a rod from hip to knee (about 15") with three screws to hold it. It has never set anything off. Wierd.

I was mad because it's not magnetic, and therefore, i couldn't stick fridge magnets to my leg.

km-when you can't walk for 4 months you amuse yourself-cm

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Of course this land is dangerous! All of the animals are capably murderous. Especially the penguins.

i'm a figment of my own imagination, sometimes i don't exist

Posts: 1099 | From: Kitsap County, WA | Registered: Oct 2002  |  IP: Logged | Report this post to a moderator
   

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