October 26, 2011 • 31 mins
What are the three types of metal detectors? How do metal detectors use electromagnetism to detect stuff? What kind of stuff can create interference when using a metal detector? Tune in as Jonathan and Chris explore the technology behind metal detectors.
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Episode Transcript
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Speaker 1 (00:00):
Brought to you by the reinvented two thousand twelve Camray.
It's ready. Are you get in touch with technology with
tech Stuff from how stuff works dot com. Hello again, everyone,
Welcome to tech stuff. My name is Chris Polett and
I'm an editor at how stuff works dot com. I
(00:22):
worked with a person across from me pretty often. Uh.
He has a senior writer at how stuff works dot com. Also,
his name is Jonathan Strickland. New Wave tried to kill
the Metal but they failed as they were stricken down
to the ground. The second time I've quoted this band. Really,
at least it isn't from Tron, that's true. Not from Tron,
(00:46):
not from war games. Um. So today we are going
to talk about you know what, This was a topic,
uh that I don't know that anyone has ever requested. Um.
It came to me actually as I was looking through
stuff in my basement and went metal detectors you should
talk about We've never talked about metal detective Actually, we've
(01:07):
sort of talked about metal detectors because metal detectors use
a lot of the technology we talked about before, especially
the relationship of magnetism to electricity. Yeah, um, but yeah,
I mean it's something that we run into very often,
especially if we travel and walk through the metal detectors
at the airport. If we're not getting back scattered or
(01:28):
uh you know, dropping stuff that you can't find, losing
your wedding ring and the maltch outside. Yikes, that's good stuff.
I've done that. I still have it, so there you go.
I've never lost my wedding ring. Alright, Well it was
only temporary. So anyway, metal detectors talk about what they are.
We're mostly going to be talking about the kind that
(01:51):
you would you would carry with you, you know, the
portable metal detectors that you would use when looking for say,
loose change on the beach, or you know, searching for
that rare civil war uh bullet or a gun that
might be part of you know, in an area that
you knew the battle happened here, so there may very
(02:12):
well be stuff here. Yeah, we we should definitely talk
about some of the applications of the technology. UM and
I would like to point out that we have an
excellent article on how stuff works dot com if you're
interested in learning more. UM it's called how metal Detectors Work. Yeah,
it should be pretty easy for people to find, written
by Jeff Tyson, and it is a very good article
(02:32):
and very uh simple to follow, easy to understand. I
would say this is this is one of those really
great how stuff works articles. Yeah, by the time you
get through it and you're like, oh, yeah, no, that
totally makes sense, especially if you have a little bit
of grounding in the way electromagnetism works and conductors and inducers.
(02:55):
So we'll we'll cover that. I mean, we've talked about
an in previous podcast, but we'll will will do a
quick brush up so that we can explain it. Um,
so you wanna, let's let's talk about the the Jeff
does a great job at breaking down what the typical
portable metal detector, the four main parts of one, what
they would be. Yeah, I mean, if you've ever seen
(03:17):
people walking around with a metal detector, you you sort
of know what they look like. They have a round
dish part that's used close to the ground, right, that's uh,
that's called the coil. Yes, and then you have the stabilizer, um,
which is the part that you actually hold onto, that
holds onto your rope, that that you see on your
(03:37):
arm because you know, frankly having used a metal detector,
it's not the lightest thing, right, This this helps, This
helps distribute the weight somewhat so that you're not you
don't feel like you're carrying around a you know, ten
pound brick the whole time, because without that then you
would get very tired, very quickly. Yeah. And the the
ten pound the aforementioned ten pound brick, which probably isn't
(03:59):
ten pounds, but it's certainly can feel that way after
a while. That's the control box, uh, And you've got
all your your electronics in there. And and then the
connect yes, the long skinny part that's the shaft. Yes,
it connects the coil at the base to the part
that you're holding. And the of course the control box.
So those are your four basic parts of your basic
(04:21):
metal detector. Yes. And then you know you have your
headphones because this is one of our episodes of stuff
that beeps and stuff what beat? Stuff? What beeps? And
it it does in fact beep. Yeah, there's usually some
sort of audible alert that you have in fact detected metal. Yeah. Now,
more sophisticated uh, metal detectors will tell you a lot
(04:43):
more often with uh, well, sometimes with with an audible tone,
but also with a visual uh, some kind of visual analysis.
We can get into that too in a minute. So
there are three main types of metal detectors, the ones
that work, the three three main approaches to to metal detection.
(05:03):
Let's say, Uh, there's UM, there's the very low frequency,
which is probably the most um, the most popular or
the most common type of MEL detector, at least for
the portable metal detectors. UM. Yeah. When we get into this,
you'll you'll see that each of these different types has
its own strengths and weaknesses. And you might say, well,
the VLF detector sounds great, why would you use the others?
(05:27):
We'll get into that. The other two are pulse induction
and beat frequency oscillation. Yeah. I I knew a beat
frequency oscillator once. He was awesome. Yeah, Anyway, I think
something else, I guess. So that's the beat boxer frequency oscillator.
(05:47):
That the uh, the VLF metal detector is the type
that I have. UM and uh. The the basic differences
uh and these as you'll see those really in the coils,
to be honest, because the VLF has two different coils.
One transmits UM and and the other is basically an
(06:11):
antenna to pick up the signals, so they're shielded from
one another so it doesn't pick up because otherwise you
turn it on, you would just have the receiver saying,
oh my gosh, there's metal everywhere. Holy crap, you're rich,
even up in the air. It's it's literally everywhere. Yeah,
that would that would not be very useful. And taking
this back to the store where I bought it. It's
(06:32):
a detector detector. This does not do me any good.
It's a detector. It's a detected or where's perry? So
the the the transmitter coil um it's it. Think of
a circle, all right. It's a this circular part at
the base of the metal detector. And if you were
(06:54):
to take it apart, which I do not recommend you do,
you would see that there is a warrant point warranty
and it may not work anymore. You will have a
there's a coil of wire that goes around the perimeter
of this circle, like it's it's wrapped around. So the
circle itself is let's say it's like a quarter of
an inch thick, and you've got this coil of wire
that's that's coiled around this quarter of an inch all
(07:16):
the way around the perimeter of this circle. When you
run electricity through this coil of wire, what happens besides
the fact that electrons are moving through because of the
nature of electricity and magnetism, Well, when you run electricity
around a coil like that, it generates a magnetic field.
That's right. It does generate a magnetic field, and it
(07:38):
generates a magnetic field perpendicular to the direction of the electricity.
It's just the nature of electromagnetism. It's amazing, it's awesome.
That's what how electro magnets work. And uh dynamo's work
in uh and sort of using this in reverse. Um.
So you've got electricity flowing through these coils and that
generates the magnetic field. Well, it doesn't just the electricity.
(08:01):
Actricity doesn't just flow in one direction, no, it it
goes back and forth. It alternates very very very fast,
almost as fast as I waffle back and forth on
what I want to eat for lunch. And that means
that the magnetic field also alternates. The direction of the
magnetic field changes. Uh. And if you think of it
(08:22):
in terms of the disc, the coil disc being towards
the ground, you can think of it as the magnetic
field occasionally is pushing against or pulling the ground in general.
That just think of it that way. That's the direction
of the magnetic field while the electricity is going around
and around in a circle, back and forth and in
the various directions, and that alternation is happening hundreds of
(08:45):
times per second. So you've got this magnetic field that's
that's just oscillating essentially right there against the ground. When
it comes to contact with something that is metal, well,
the metal will start to that magnetic field will as
a a weaker magnetic field to generate around the metal object. Yeah.
(09:05):
Basically induces a magnetic field in a metal object in
the ground. So let's say, uh, um, you drop a
screw and when you're trying to build something outside and
you're trying to find it with your metal detector so
that you don't step on it with a bare foot
and you know need a trip to the hospital and
a technis shot. Um. So as soon as you run
(09:27):
the coil of the field and you're not trying to
touch the ground with it, you really just need it
close to the ground. Um, it will find the screw.
The screw will basically start inducing a it will end
metal detected detector will generate a field, and that will
cause the screw to generate its own a weaker magnetic
(09:47):
Oh yeah, yeah, considerably weaker, but it will the metal detector, Uh,
the antenna coil inside the transmitter coil will say, oh,
look there's something right there that is generating a magnetic field.
It's right here. And the way that the the that
antenna works is the way I was talking about with
a dynamo. So if you have a coil of wire
(10:08):
and you introduce it into a fluctuating magnetic field, that
will induce the flow of electricity within the wire. Just
as electricity and the wire will create generate a magnetic field,
a magnetic field will induce electricity to run through wire
fluctuating magnetic field. It can't just be just a magnetic field.
So when the weak magnetic field from the mental object
(10:32):
hits that that the wire in the the the the
receiver coil that generates the electricity, which then alerts the
control box that that's what When the control box gets
an electric signal from the receiver coil that says beep,
something is here stuff? What beeps? Yeah? Stuff what beeps. So, yeah,
(10:55):
that's the basic uh way that a VLF metal detector works.
It shoots out this magnetic field. The polar polarization is
alternating the same as the the flow of electricity around
the the the transmitter coil and then the receiver coil
picks up any magnetic fields generated by metal objects that
(11:17):
are under the surface of the ground. It's pretty cool
and generally it will go more or less about a foot. Yeah,
you're not going to find you know, the Earth's core
and the depth of what the depth of the The
effectiveness of this metal detector is also going to be
dependent on other factors, like the nature of the metal
(11:41):
object itself, because some some metal objects are going to
generate a stronger magnetic field than others, like iron. Uh.
It will also depend on the composition of the soil.
So the soil has a lot of elements in it
that would create a magnetic field, you're gonna get a
lot of interference, yeah, or or an environment where uh
(12:04):
that that would conduct electricity more readily. Like if you're
trying to look for something and say saltwater, you might
have more difficulty than if you were looking you know,
on on a different type of soil. UM. Now there
is something to uh called phase shifting, and that's how
it's able to tell what sort of metal, what sort
(12:25):
of metal it is UM, And it really has to
do with the the inductance and the resistance of the
object that you're looking for. Now, the inductance is UM
the objects ability to conduct electricity UM. And if it's inductive,
it would be able to to conduct electricity easily, but
it's slow to react to changes in the electrical current. UM.
(12:49):
If it's resistive, it doesn't conduct electricity is easily, but
it is quick to react to those those changes. And
a sophisticated metal detector will be able to tell you, well,
that's I think that's gold or based upon that phase shift.
Because the face shift for a particular element is going
to remain constant, you know. That's that's why if it
(13:09):
didn't remain constant, then this wouldn't matter at all, because
it wouldn't you know, we wouldn't know, you know, it's
like we just know something is down there, we wouldn't
know have any way of guessing what it is. But
because it's an inherent property of this particular material, and
it's really dependent upon the molecular composition of whatever that
material is. UM, then we can make start making guesses
(13:31):
based on that phase shift UM. And it's yeah, the
face shift, you know, with it being that difference in
timing between the transmitters frequency uh and the frequency of
the magnetic wave that's generated by the object. Um. You know.
It's these are sort of ranges, right like if it
if it's a certain range there, then the male detector
(13:54):
can give a best guess as to what that material maybe.
So there's no ever necessarily a guarantee that whatever it
is you've detected is specifically going to be you know,
aluminum versus goal or whatever. But it's a it's an indicator,
so that uh, it gives you at least the idea
of you know, is that you measure risk versus reward, right,
(14:17):
the risk of exhausting yourself digging up useless junk versus
the reward of possibly finding something that is of true value. Yes,
so that's really the whole purpose of this anyway. It's
just you know, to kind of tune in on that
beep and make it more meaningful so that you know,
you don't waste your time digging up uh little poll
(14:38):
tabs from soda cans. Yeah. Now, yeah, if you have
a higher inductance, it's going to create a larger phase shift,
and lower inductance has a smaller phase shift. Now, metal
detectors use a phase demodulator to read this, and uh,
what that creates is scrimination. Yeah, yeah, I know it's terrible,
(15:03):
but within metal detectors, it's okay, yeah, because the thing is,
a good metal detector will allow you to filter out
stuff that you don't want because the metal detector, if
you said it at a certain setting and say, you know,
I'm really looking for metals that fall within this range, yeah,
perhaps gold. I've actually seen uh, metal detector booths and
(15:27):
they'll have like this giant jar of class rings that
people have lost, which is sort of a you know,
reminder that you know, hey, you you can find stuff
if you just take your metal. Of course people look
at you funny, but yeah, that way. That way. Let's
say that you're out there looking for something specific, you
can you know, some meld detectors have settings where you can, uh,
(15:47):
you can tell the meld detector to ignore anything outside
of a particular range of frequencies. So in other words,
you're saying. You might say, I'm only interested in finding silver.
Let's say, So what I'll do is I'll set my
MELD detector so that it'll it'll only uh activate, it
will only alert me if something that generates a magnetic
(16:09):
frequency similar to that of silver is encountered by the
metal detector. So in other words, what you might you
might pass over a gold deposit and nothing happens. But
as soon as you go over that that silver half
dollar but ink, and then you dig it up and
you're like, hey, look I'm rich. I Meanwhile, the gold
guy behind you was like, sucker. Well that's that. That
(16:30):
can be called notching. Basically, you set it for a
if you imagine a scale, uh you know, one notch
on that scale, or some of the more sophisticated metal
detectors will allow you to set more than one notch r.
So that way you could look for both silver and
gold and you'd say, ha ha, Mr smarty gold pants. Um.
Now that's that's the VLF. The post induction metal detector
(16:55):
is a little bit different. Yeah, it uses only a
single coil, or it can use multiple oils working at
the same time. Yeah, but there these are these are
coils that haven't that don't have that shielding. It's not
like you have a transmitter coil and then a receiver coil,
and all of these coils are generating very quick bursts
(17:16):
of electricity, which of course creates a very sharp magnetic wave.
And uh so it's just a really quick spike. And
then what will happen is that magnetic wave if it
encounters a metal object. Again, that metal object will have
to will generate its own magnetic wave, and the metal
detector will pick up on that, and it's all it's
kind of like an echo, right, It's kind of like
(17:38):
an echo, right, thank you. So if you think about
it that way, it's it's like sending out a signal
and waiting for something to bounce back. Yeah, but it's
very fast. I mean the average is about a hundred
pulses per second, which is yeah, fairly rapid, pretty quick.
And then you're looking for the reflected pulse, yes, which
(17:59):
is what the the object that is under the ground
or wherever. Uh, that's the the the pulse that it
generates in response to the one that came from the
mel detector. Yeah, Now it uses a these these types
of metal detectors use a sampling circuit, basically trying to
determine whether it's actually metal that you're finding or something else,
(18:21):
some other type of magnetic field that is interfering with
this signal, and it's basically judging that based on the
amount of time it takes UH two to reflect. Um. Yeah,
that's another thing that we should point out is that
MEL detectors can and encounter lots of different kinds of interference.
Not just the composition of the soil. But let's say
(18:41):
that you happen to cross over where an electrical cable
has been buried. That could cause a problem. Your male
detector might think that, hey, look there's gold in them
the Oar hills, and truth, what you've hit is the
power line, which you don't want to dig that up. No,
it could result in everything from death, which UH is
(19:03):
in the metal detection biz we call a bad thing,
or it could result in or and or it could
result in an entire region losing electricity. That's also a
bad thing. That's that's a very good point. I mean,
these these metal detectures are not only incredibly useful, they
can be a lot of fun. A lot of hobbyists
(19:25):
love them, but you do have to be very very careful. Um.
And it's good to know your the territory to quote
a certain musical, but yeah, I mean you need to
know the territory. You gotta know where you're you're digging
because you you really could get hurt. Honestly, I've got
Rock Island going through my head. Now, cash for the merchandise,
(19:47):
al right, sorry, all right. So we have an integrator
on these, on these pulse induction metal detectives, uh, and
that's reading the signals from the sampling circuit and converting
them to D that's current Yeah, not the district of Columbia. Yeah,
not not a comic book publisher either. And that's feeding
into it the audio circuit and that is what beats. Yeah,
(20:10):
that's the stuff what beats for the pulse indicator. The
thing is, uh, these are very good at detecting metal,
but they're not very good at discriminating between different types
of metal. Um, So you're not going to get the
level of precision that you would get with a VLF
metal detector. But you know they also you've got to
(20:30):
remember that these different types of detectors, while some of
them may work in conditions that others do not. There's
there's also a price range here as well. Oh yes, yes, now, yeah,
the beat frequency oscillator metal detectors can be uh fairly affordable. Um.
They also used two coils, but one is in the
search head and the other one is actually in the
(20:51):
control box. UM. They're connected to an oscillator that as
pulsing UM many many times per second. That was in
of times per second UM. And and the and the
frequency of the one that's inside the control box and
the one that's inside the actual detector are slightly offset.
They're not in sync or backstreet boys. Oh you got
(21:16):
the other one I was gonna do U. So yeah, well,
ironically enough to our our current part of the conversation.
They create radio waves um, and then the receiver in
the control box uh picks that up and plays tones
to let you know when you're you're coming into range
(21:37):
of metal, right, Because what's happening is that when the
metal encounters this uh, this this this electromagnetic field that
is going to alter that radio wave. And so what's
really listening for our changes in the sound that you're hearing.
The change in the sound indicates that you have encountered
something that is generating this man medic field because of
(22:00):
the one you're generating, they know what they're going through. Um.
So yeah, I mean detectors killed the radio star. But
these these uh, Like I said, these are a lot
cheaper to manufacture. And therefore, if you are uh interested
in getting your feet wet, uh and the metal detector
(22:21):
the thing is a hobby, you might consider this, but
they're they're far less accurate in terms of um being
able to give you any kind of discrimination or any
really any real control. Um they actually use these are
the types of metal detectors that you the handheld detectors
that they use the airport. Um, you know, if you
walk through the scanner and and then yeah, that's the
(22:43):
type they use in the one because they don't really
need to know whether it's gold or iron. They just
want to know what it is that you've got now
once once, once something beeps, then they can say, all right,
well let's see what that is. They don't, you know,
necessarily need to know immediately. Yeah. And the metal detectors
the frames at the airport, they work on essentially the
(23:03):
same principles we've talked about here. They're generating electromagnetic fields,
which of course has brought up some concerns with people
who may have UH devices like medical devices that could
react to electromagnetic fields. That's why you'll occasionally hear about
UH folks like that having to go through like a
pat down security as opposed to a metal detector, just
(23:25):
because of the concern that it could interfere with a
medical device. Right right now, with with all of these um,
what you're gonna find depends on the type of metal detector,
the type of metal that you're looking for, about how
big it is, the kind of makeup of the soil,
um any kind of interference from other objects, and the halo,
(23:46):
which is basically, you know, if you have a piece
of metal in the soil, it's going to read a
little bit differently, it's going to it's going to register
is larger than it actually is. Yeah, because those interesting things, Yeah,
they I could see your halo. I was I was waiting.
I was trying so hard not to go into that.
We've done so many songs references in this podcast. It's crazy.
(24:10):
And this might be our like our most intense references
per minute other than the first one. None of them
have been metal. That's true. That's true. So, but yeah,
now that it is, really the halo effect is really
an interesting thing. It's just this the fact that certain
kinds of metal objects can actually increase the conductivity of
(24:33):
the soil that's around them. And the longer that object,
those particular objects have been in the ground, the greater
that conductivity change can be. So then yeah, you might
be like, hey, there's a there's a Sherman tank down
there or something, and it just turns out that there's
another object that's been down there and has affected the
conductivity of the soil. Um that's pretty phenomenal. If you
(24:54):
have a Sherman tank buried in your backyard, please give
me a call. I'm I'm interested to well, not anymore. Yeah,
I mean, well, you think about it. It It makes sense
if you have let's say, let's say the aforementioned screw
and it's been there for a couple of years. It's
let's say iron, and it's beginning to rust, it's beginning
to disintegrate, and you can change the conduct conductivity depending
(25:16):
on the Again, it also depends it's rain then, you know, yeah,
it really depends upon the composition of the soil. Itself
as well. So, yeah, if there's a lot of iron
content in the soil, that's gonna cause some issues when
you're when you're doing metal detection. You know, you're just
you're gonna get a lot of interference and you just
have to factor that in. Doesn't mean that the metal
detector is going to be useless. It just means that
(25:36):
you have to kind of adjust your expectations and listen
for bigger changes in UH in the soil content and
not and kind of kind of ignore the noise, separate
the signal from the noise. Yeah, I would argue that
it probably takes some personal discrimination too. You really need
to get out there in practice if you're going to
(25:56):
start using doing metal detecting as a hobby, um, and
maybe even trying it with things that you what that
you know what they are, you know, some coins, a ring,
a screwdriver, you know, something that hammers, you know, Sherman tanks,
the stuff that you do, you know, tend to run
across in your general area. Well, you know, you never
(26:17):
know what you're gonna find when you're out in the
field somewhere, and if you can identify what they are,
you can get a sense of what different metals sound like.
How get a feel for how your metal detector works,
how sensitive it is, and how sensitive it is. Um, yeah,
it can be a lot of fun to uh, as
you were talking about before, like going to uh battlefields.
(26:37):
Of course, I imagine probably all the major battlefields have
been picked pretty clean at this point, you think, But
it's it's amazing once you can stumble across It's again,
it's one of those things where it may be it
may require a phenomenal amount of luck. Sure experience, there's
no such thing as luck, but it may require a
(26:58):
phenomenal a series of incidences in order for you to
stumble across the right section to find something significant. But
really you never know. You just it's it's amazing to
me how something that you would think, oh, well, this
has got so much traffic, there's no way there's anything
of significance can be found there. And yet like five
years later you'll find that the find of this injury.
(27:20):
How did that happen? But yeah, and you don't know
what's around in places that you know might be a value.
I mean again, people find I'm still searching for black
Beard's treasure. All kinds of stuff out in time, I
go to the Carolinas out in normal places. Maybe I
should actually go to the everyday beach instead of in London.
(27:41):
That's probably part of the problem. Yeah, you know, Charleston
is so nice. I just like to, you know, spend
some time there enough. Yeah, people even use them for prospecting,
you know, and uh and for coin shooting. Now, if
you do want to use your mail detective for prospecting,
there are some things you need to remember. First, you
have to grow a fear of at least a foot
(28:04):
in length. That needs to be gray and grizzly. He's
gonna punchy ladies and gentlemen. Second, you need to be
able to say things like consarnate with a straight face.
And then third, the most important thing about being a
prospector is you have to develop your crazy prospect or
dance when you strike gold. Very important. Yeah. Coin shooting
is uh the practice of looking for coins after an
(28:25):
outdoor event like a wedding or or like stock you
go to you go to some big outdoor conference and
you take a male detector and you start looking for
for change. The thing is that if you live in
an area where this sort of stuff goes on in
a regular basis, you could presumably pay off that mel
detector after you know, a few visits to these these
(28:47):
areas once the concert is over or a game or whatever. Um,
you know. And and because we're usually talking about change,
you know, some some nickels or dimes or quarters or whatever,
but whatever money you use, because you know, it's not
all just United States folks. Well, if you find euros,
you're rich, because I think a euro is equivalent to
(29:10):
like seventeen dollars at this point. Um, that's a gross exaggeration,
but the point being that, you know, yeah, you could
conceivably pay off your mail detectors costs after a few visits,
you know. Um. But although a lot of people just
enjoy the this as a hobby, like it's just it's
(29:30):
a way to get outside. You're enjoying your joying time
outdoors and you're finding stuff which just brings you a
level of of of satisfaction, right. It means that whole
treasure hunting aspect, you know, when I when we discover something,
there's an element of satisfaction there that you can't put
a monetary value on. Yeah, yeah, you're just like, hey,
(29:51):
look I found some money woo, And yeah, there's there's
an enjoyment level there that goes beyond. Oh and it's
worth you know, seventy five cents. Yes, um, although the cents,
it's always nice to have seventy five cents more than
you had, but when you showed up at the field.
So yeah, I keep wondering, actually, why the value of
(30:12):
the giant jar of class rings. You'd figure they'd cash
that in. Yeah, I mean for more than just selling
metal detectors. Maybe that's more lucrative than selling off some
of those classroom you have, prying out all the little
jewels and melting that stuff down and then just hey,
you know, how can yourself bar a gold? It's worth
its weight in gold. Anyway, I need to go out
and get a mel detector. Yeah they're fun, Yeah they
(30:35):
could be. They could be a lot of fun. Yeah, definitely.
So anyway, that that kind of sums up this podcast
on how mel detectors work. Remember, we do have an
article on the website that goes into more detail and
kind of walks you through what we've talked about here.
I highly recommend it. If you guys have any technology
you would want us to talk about, please let us know.
(30:55):
You can drop us a line on Facebook or Twitter,
are handled there as tech Stuff hs W, or send
us an email. That address is tech stuff at how
stuff works dot com. Chris and I will talk to
you again beep really soon. Be sure to check out
our new video podcast, Stuff from the Future. Join how
Stuff Work staff as we explore the most promising and
(31:17):
perplexing possibilities us tomorrow. The How Stuff Works iPhone app
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you
Brought to you by the reinvented two thousand twelve Camray.
It's ready. Are you get in touch with technology with
tech Stuff from how stuff works dot com. Hello again, everyone,
Welcome to tech stuff. My name is Chris Polett and
I'm an editor at how stuff works dot com. I
(00:22):
worked with a person across from me pretty often. Uh.
He has a senior writer at how stuff works dot com. Also,
his name is Jonathan Strickland. New Wave tried to kill
the Metal but they failed as they were stricken down
to the ground. The second time I've quoted this band. Really,
at least it isn't from Tron, that's true. Not from Tron,
(00:46):
not from war games. Um. So today we are going
to talk about you know what, This was a topic,
uh that I don't know that anyone has ever requested. Um.
It came to me actually as I was looking through
stuff in my basement and went metal detectors you should
talk about We've never talked about metal detective Actually, we've
(01:07):
sort of talked about metal detectors because metal detectors use
a lot of the technology we talked about before, especially
the relationship of magnetism to electricity. Yeah, um, but yeah,
I mean it's something that we run into very often,
especially if we travel and walk through the metal detectors
at the airport. If we're not getting back scattered or
(01:28):
uh you know, dropping stuff that you can't find, losing
your wedding ring and the maltch outside. Yikes, that's good stuff.
I've done that. I still have it, so there you go.
I've never lost my wedding ring. Alright, Well it was
only temporary. So anyway, metal detectors talk about what they are.
We're mostly going to be talking about the kind that
(01:51):
you would you would carry with you, you know, the
portable metal detectors that you would use when looking for say,
loose change on the beach, or you know, searching for
that rare civil war uh bullet or a gun that
might be part of you know, in an area that
you knew the battle happened here, so there may very
(02:12):
well be stuff here. Yeah, we we should definitely talk
about some of the applications of the technology. UM and
I would like to point out that we have an
excellent article on how stuff works dot com if you're
interested in learning more. UM it's called how metal Detectors Work. Yeah,
it should be pretty easy for people to find, written
by Jeff Tyson, and it is a very good article
(02:32):
and very uh simple to follow, easy to understand. I
would say this is this is one of those really
great how stuff works articles. Yeah, by the time you
get through it and you're like, oh, yeah, no, that
totally makes sense, especially if you have a little bit
of grounding in the way electromagnetism works and conductors and inducers.
(02:55):
So we'll we'll cover that. I mean, we've talked about
an in previous podcast, but we'll will will do a
quick brush up so that we can explain it. Um,
so you wanna, let's let's talk about the the Jeff
does a great job at breaking down what the typical
portable metal detector, the four main parts of one, what
they would be. Yeah, I mean, if you've ever seen
(03:17):
people walking around with a metal detector, you you sort
of know what they look like. They have a round
dish part that's used close to the ground, right, that's uh,
that's called the coil. Yes, and then you have the stabilizer, um,
which is the part that you actually hold onto, that
holds onto your rope, that that you see on your
(03:37):
arm because you know, frankly having used a metal detector,
it's not the lightest thing, right, This this helps, This
helps distribute the weight somewhat so that you're not you
don't feel like you're carrying around a you know, ten
pound brick the whole time, because without that then you
would get very tired, very quickly. Yeah. And the the
ten pound the aforementioned ten pound brick, which probably isn't
(03:59):
ten pounds, but it's certainly can feel that way after
a while. That's the control box, uh, And you've got
all your your electronics in there. And and then the
connect yes, the long skinny part that's the shaft. Yes,
it connects the coil at the base to the part
that you're holding. And the of course the control box.
So those are your four basic parts of your basic
(04:21):
metal detector. Yes. And then you know you have your
headphones because this is one of our episodes of stuff
that beeps and stuff what beat? Stuff? What beeps? And
it it does in fact beep. Yeah, there's usually some
sort of audible alert that you have in fact detected metal. Yeah. Now,
more sophisticated uh, metal detectors will tell you a lot
(04:43):
more often with uh, well, sometimes with with an audible tone,
but also with a visual uh, some kind of visual analysis.
We can get into that too in a minute. So
there are three main types of metal detectors, the ones
that work, the three three main approaches to to metal detection.
(05:03):
Let's say, Uh, there's UM, there's the very low frequency,
which is probably the most um, the most popular or
the most common type of MEL detector, at least for
the portable metal detectors. UM. Yeah. When we get into this,
you'll you'll see that each of these different types has
its own strengths and weaknesses. And you might say, well,
the VLF detector sounds great, why would you use the others?
(05:27):
We'll get into that. The other two are pulse induction
and beat frequency oscillation. Yeah. I I knew a beat
frequency oscillator once. He was awesome. Yeah, Anyway, I think
something else, I guess. So that's the beat boxer frequency oscillator.
(05:47):
That the uh, the VLF metal detector is the type
that I have. UM and uh. The the basic differences
uh and these as you'll see those really in the coils,
to be honest, because the VLF has two different coils.
One transmits UM and and the other is basically an
(06:11):
antenna to pick up the signals, so they're shielded from
one another so it doesn't pick up because otherwise you
turn it on, you would just have the receiver saying,
oh my gosh, there's metal everywhere. Holy crap, you're rich,
even up in the air. It's it's literally everywhere. Yeah,
that would that would not be very useful. And taking
this back to the store where I bought it. It's
(06:32):
a detector detector. This does not do me any good.
It's a detector. It's a detected or where's perry? So
the the the transmitter coil um it's it. Think of
a circle, all right. It's a this circular part at
the base of the metal detector. And if you were
(06:54):
to take it apart, which I do not recommend you do,
you would see that there is a warrant point warranty
and it may not work anymore. You will have a
there's a coil of wire that goes around the perimeter
of this circle, like it's it's wrapped around. So the
circle itself is let's say it's like a quarter of
an inch thick, and you've got this coil of wire
that's that's coiled around this quarter of an inch all
(07:16):
the way around the perimeter of this circle. When you
run electricity through this coil of wire, what happens besides
the fact that electrons are moving through because of the
nature of electricity and magnetism, Well, when you run electricity
around a coil like that, it generates a magnetic field.
That's right. It does generate a magnetic field, and it
(07:38):
generates a magnetic field perpendicular to the direction of the electricity.
It's just the nature of electromagnetism. It's amazing, it's awesome.
That's what how electro magnets work. And uh dynamo's work
in uh and sort of using this in reverse. Um.
So you've got electricity flowing through these coils and that
generates the magnetic field. Well, it doesn't just the electricity.
(08:01):
Actricity doesn't just flow in one direction, no, it it
goes back and forth. It alternates very very very fast,
almost as fast as I waffle back and forth on
what I want to eat for lunch. And that means
that the magnetic field also alternates. The direction of the
magnetic field changes. Uh. And if you think of it
(08:22):
in terms of the disc, the coil disc being towards
the ground, you can think of it as the magnetic
field occasionally is pushing against or pulling the ground in general.
That just think of it that way. That's the direction
of the magnetic field while the electricity is going around
and around in a circle, back and forth and in
the various directions, and that alternation is happening hundreds of
(08:45):
times per second. So you've got this magnetic field that's
that's just oscillating essentially right there against the ground. When
it comes to contact with something that is metal, well,
the metal will start to that magnetic field will as
a a weaker magnetic field to generate around the metal object. Yeah.
(09:05):
Basically induces a magnetic field in a metal object in
the ground. So let's say, uh, um, you drop a
screw and when you're trying to build something outside and
you're trying to find it with your metal detector so
that you don't step on it with a bare foot
and you know need a trip to the hospital and
a technis shot. Um. So as soon as you run
(09:27):
the coil of the field and you're not trying to
touch the ground with it, you really just need it
close to the ground. Um, it will find the screw.
The screw will basically start inducing a it will end
metal detected detector will generate a field, and that will
cause the screw to generate its own a weaker magnetic
(09:47):
Oh yeah, yeah, considerably weaker, but it will the metal detector, Uh,
the antenna coil inside the transmitter coil will say, oh,
look there's something right there that is generating a magnetic field.
It's right here. And the way that the the that
antenna works is the way I was talking about with
a dynamo. So if you have a coil of wire
(10:08):
and you introduce it into a fluctuating magnetic field, that
will induce the flow of electricity within the wire. Just
as electricity and the wire will create generate a magnetic field,
a magnetic field will induce electricity to run through wire
fluctuating magnetic field. It can't just be just a magnetic field.
So when the weak magnetic field from the mental object
(10:32):
hits that that the wire in the the the the
receiver coil that generates the electricity, which then alerts the
control box that that's what When the control box gets
an electric signal from the receiver coil that says beep,
something is here stuff? What beeps? Yeah? Stuff what beeps. So, yeah,
(10:55):
that's the basic uh way that a VLF metal detector works.
It shoots out this magnetic field. The polar polarization is
alternating the same as the the flow of electricity around
the the the transmitter coil and then the receiver coil
picks up any magnetic fields generated by metal objects that
(11:17):
are under the surface of the ground. It's pretty cool
and generally it will go more or less about a foot. Yeah,
you're not going to find you know, the Earth's core
and the depth of what the depth of the The
effectiveness of this metal detector is also going to be
dependent on other factors, like the nature of the metal
(11:41):
object itself, because some some metal objects are going to
generate a stronger magnetic field than others, like iron. Uh.
It will also depend on the composition of the soil.
So the soil has a lot of elements in it
that would create a magnetic field, you're gonna get a
lot of interference, yeah, or or an environment where uh
(12:04):
that that would conduct electricity more readily. Like if you're
trying to look for something and say saltwater, you might
have more difficulty than if you were looking you know,
on on a different type of soil. UM. Now there
is something to uh called phase shifting, and that's how
it's able to tell what sort of metal, what sort
(12:25):
of metal it is UM, And it really has to
do with the the inductance and the resistance of the
object that you're looking for. Now, the inductance is UM
the objects ability to conduct electricity UM. And if it's inductive,
it would be able to to conduct electricity easily, but
it's slow to react to changes in the electrical current. UM.
(12:49):
If it's resistive, it doesn't conduct electricity is easily, but
it is quick to react to those those changes. And
a sophisticated metal detector will be able to tell you, well,
that's I think that's gold or based upon that phase shift.
Because the face shift for a particular element is going
to remain constant, you know. That's that's why if it
(13:09):
didn't remain constant, then this wouldn't matter at all, because
it wouldn't you know, we wouldn't know, you know, it's
like we just know something is down there, we wouldn't
know have any way of guessing what it is. But
because it's an inherent property of this particular material, and
it's really dependent upon the molecular composition of whatever that
material is. UM, then we can make start making guesses
(13:31):
based on that phase shift UM. And it's yeah, the
face shift, you know, with it being that difference in
timing between the transmitters frequency uh and the frequency of
the magnetic wave that's generated by the object. Um. You know.
It's these are sort of ranges, right like if it
if it's a certain range there, then the male detector
(13:54):
can give a best guess as to what that material maybe.
So there's no ever necessarily a guarantee that whatever it
is you've detected is specifically going to be you know,
aluminum versus goal or whatever. But it's a it's an indicator,
so that uh, it gives you at least the idea
of you know, is that you measure risk versus reward, right,
(14:17):
the risk of exhausting yourself digging up useless junk versus
the reward of possibly finding something that is of true value. Yes,
so that's really the whole purpose of this anyway. It's
just you know, to kind of tune in on that
beep and make it more meaningful so that you know,
you don't waste your time digging up uh little poll
(14:38):
tabs from soda cans. Yeah. Now, yeah, if you have
a higher inductance, it's going to create a larger phase shift,
and lower inductance has a smaller phase shift. Now, metal
detectors use a phase demodulator to read this, and uh,
what that creates is scrimination. Yeah, yeah, I know it's terrible,
(15:03):
but within metal detectors, it's okay, yeah, because the thing is,
a good metal detector will allow you to filter out
stuff that you don't want because the metal detector, if
you said it at a certain setting and say, you know,
I'm really looking for metals that fall within this range, yeah,
perhaps gold. I've actually seen uh, metal detector booths and
(15:27):
they'll have like this giant jar of class rings that
people have lost, which is sort of a you know,
reminder that you know, hey, you you can find stuff
if you just take your metal. Of course people look
at you funny, but yeah, that way. That way. Let's
say that you're out there looking for something specific, you
can you know, some meld detectors have settings where you can, uh,
(15:47):
you can tell the meld detector to ignore anything outside
of a particular range of frequencies. So in other words,
you're saying. You might say, I'm only interested in finding silver.
Let's say, So what I'll do is I'll set my
MELD detector so that it'll it'll only uh activate, it
will only alert me if something that generates a magnetic
(16:09):
frequency similar to that of silver is encountered by the
metal detector. So in other words, what you might you
might pass over a gold deposit and nothing happens. But
as soon as you go over that that silver half
dollar but ink, and then you dig it up and
you're like, hey, look I'm rich. I Meanwhile, the gold
guy behind you was like, sucker. Well that's that. That
(16:30):
can be called notching. Basically, you set it for a
if you imagine a scale, uh you know, one notch
on that scale, or some of the more sophisticated metal
detectors will allow you to set more than one notch r.
So that way you could look for both silver and
gold and you'd say, ha ha, Mr smarty gold pants. Um.
Now that's that's the VLF. The post induction metal detector
(16:55):
is a little bit different. Yeah, it uses only a
single coil, or it can use multiple oils working at
the same time. Yeah, but there these are these are
coils that haven't that don't have that shielding. It's not
like you have a transmitter coil and then a receiver coil,
and all of these coils are generating very quick bursts
(17:16):
of electricity, which of course creates a very sharp magnetic wave.
And uh so it's just a really quick spike. And
then what will happen is that magnetic wave if it
encounters a metal object. Again, that metal object will have
to will generate its own magnetic wave, and the metal
detector will pick up on that, and it's all it's
kind of like an echo, right, It's kind of like
(17:38):
an echo, right, thank you. So if you think about
it that way, it's it's like sending out a signal
and waiting for something to bounce back. Yeah, but it's
very fast. I mean the average is about a hundred
pulses per second, which is yeah, fairly rapid, pretty quick.
And then you're looking for the reflected pulse, yes, which
(17:59):
is what the the object that is under the ground
or wherever. Uh, that's the the the pulse that it
generates in response to the one that came from the
mel detector. Yeah, Now it uses a these these types
of metal detectors use a sampling circuit, basically trying to
determine whether it's actually metal that you're finding or something else,
(18:21):
some other type of magnetic field that is interfering with
this signal, and it's basically judging that based on the
amount of time it takes UH two to reflect. Um. Yeah,
that's another thing that we should point out is that
MEL detectors can and encounter lots of different kinds of interference.
Not just the composition of the soil. But let's say
(18:41):
that you happen to cross over where an electrical cable
has been buried. That could cause a problem. Your male
detector might think that, hey, look there's gold in them
the Oar hills, and truth, what you've hit is the
power line, which you don't want to dig that up. No,
it could result in everything from death, which UH is
(19:03):
in the metal detection biz we call a bad thing,
or it could result in or and or it could
result in an entire region losing electricity. That's also a
bad thing. That's that's a very good point. I mean,
these these metal detectures are not only incredibly useful, they
can be a lot of fun. A lot of hobbyists
(19:25):
love them, but you do have to be very very careful. Um.
And it's good to know your the territory to quote
a certain musical, but yeah, I mean you need to
know the territory. You gotta know where you're you're digging
because you you really could get hurt. Honestly, I've got
Rock Island going through my head. Now, cash for the merchandise,
(19:47):
al right, sorry, all right. So we have an integrator
on these, on these pulse induction metal detectives, uh, and
that's reading the signals from the sampling circuit and converting
them to D that's current Yeah, not the district of Columbia. Yeah,
not not a comic book publisher either. And that's feeding
into it the audio circuit and that is what beats. Yeah,
(20:10):
that's the stuff what beats for the pulse indicator. The
thing is, uh, these are very good at detecting metal,
but they're not very good at discriminating between different types
of metal. Um, So you're not going to get the
level of precision that you would get with a VLF
metal detector. But you know they also you've got to
(20:30):
remember that these different types of detectors, while some of
them may work in conditions that others do not. There's
there's also a price range here as well. Oh yes, yes, now, yeah,
the beat frequency oscillator metal detectors can be uh fairly affordable. Um.
They also used two coils, but one is in the
search head and the other one is actually in the
(20:51):
control box. UM. They're connected to an oscillator that as
pulsing UM many many times per second. That was in
of times per second UM. And and the and the
frequency of the one that's inside the control box and
the one that's inside the actual detector are slightly offset.
They're not in sync or backstreet boys. Oh you got
(21:16):
the other one I was gonna do U. So yeah, well,
ironically enough to our our current part of the conversation.
They create radio waves um, and then the receiver in
the control box uh picks that up and plays tones
to let you know when you're you're coming into range
(21:37):
of metal, right, Because what's happening is that when the
metal encounters this uh, this this this electromagnetic field that
is going to alter that radio wave. And so what's
really listening for our changes in the sound that you're hearing.
The change in the sound indicates that you have encountered
something that is generating this man medic field because of
(22:00):
the one you're generating, they know what they're going through. Um.
So yeah, I mean detectors killed the radio star. But
these these uh, Like I said, these are a lot
cheaper to manufacture. And therefore, if you are uh interested
in getting your feet wet, uh and the metal detector
(22:21):
the thing is a hobby, you might consider this, but
they're they're far less accurate in terms of um being
able to give you any kind of discrimination or any
really any real control. Um they actually use these are
the types of metal detectors that you the handheld detectors
that they use the airport. Um, you know, if you
walk through the scanner and and then yeah, that's the
(22:43):
type they use in the one because they don't really
need to know whether it's gold or iron. They just
want to know what it is that you've got now
once once, once something beeps, then they can say, all right,
well let's see what that is. They don't, you know,
necessarily need to know immediately. Yeah. And the metal detectors
the frames at the airport, they work on essentially the
(23:03):
same principles we've talked about here. They're generating electromagnetic fields,
which of course has brought up some concerns with people
who may have UH devices like medical devices that could
react to electromagnetic fields. That's why you'll occasionally hear about
UH folks like that having to go through like a
pat down security as opposed to a metal detector, just
(23:25):
because of the concern that it could interfere with a
medical device. Right right now, with with all of these um,
what you're gonna find depends on the type of metal detector,
the type of metal that you're looking for, about how
big it is, the kind of makeup of the soil,
um any kind of interference from other objects, and the halo,
(23:46):
which is basically, you know, if you have a piece
of metal in the soil, it's going to read a
little bit differently, it's going to it's going to register
is larger than it actually is. Yeah, because those interesting things, Yeah,
they I could see your halo. I was I was waiting.
I was trying so hard not to go into that.
We've done so many songs references in this podcast. It's crazy.
(24:10):
And this might be our like our most intense references
per minute other than the first one. None of them
have been metal. That's true. That's true. So, but yeah,
now that it is, really the halo effect is really
an interesting thing. It's just this the fact that certain
kinds of metal objects can actually increase the conductivity of
(24:33):
the soil that's around them. And the longer that object,
those particular objects have been in the ground, the greater
that conductivity change can be. So then yeah, you might
be like, hey, there's a there's a Sherman tank down
there or something, and it just turns out that there's
another object that's been down there and has affected the
conductivity of the soil. Um that's pretty phenomenal. If you
(24:54):
have a Sherman tank buried in your backyard, please give
me a call. I'm I'm interested to well, not anymore. Yeah,
I mean, well, you think about it. It It makes sense
if you have let's say, let's say the aforementioned screw
and it's been there for a couple of years. It's
let's say iron, and it's beginning to rust, it's beginning
to disintegrate, and you can change the conduct conductivity depending
(25:16):
on the Again, it also depends it's rain then, you know, yeah,
it really depends upon the composition of the soil. Itself
as well. So, yeah, if there's a lot of iron
content in the soil, that's gonna cause some issues when
you're when you're doing metal detection. You know, you're just
you're gonna get a lot of interference and you just
have to factor that in. Doesn't mean that the metal
detector is going to be useless. It just means that
(25:36):
you have to kind of adjust your expectations and listen
for bigger changes in UH in the soil content and
not and kind of kind of ignore the noise, separate
the signal from the noise. Yeah, I would argue that
it probably takes some personal discrimination too. You really need
to get out there in practice if you're going to
(25:56):
start using doing metal detecting as a hobby, um, and
maybe even trying it with things that you what that
you know what they are, you know, some coins, a ring,
a screwdriver, you know, something that hammers, you know, Sherman tanks,
the stuff that you do, you know, tend to run
across in your general area. Well, you know, you never
(26:17):
know what you're gonna find when you're out in the
field somewhere, and if you can identify what they are,
you can get a sense of what different metals sound like.
How get a feel for how your metal detector works,
how sensitive it is, and how sensitive it is. Um, yeah,
it can be a lot of fun to uh, as
you were talking about before, like going to uh battlefields.
(26:37):
Of course, I imagine probably all the major battlefields have
been picked pretty clean at this point, you think, But
it's it's amazing once you can stumble across It's again,
it's one of those things where it may be it
may require a phenomenal amount of luck. Sure experience, there's
no such thing as luck, but it may require a
(26:58):
phenomenal a series of incidences in order for you to
stumble across the right section to find something significant. But
really you never know. You just it's it's amazing to
me how something that you would think, oh, well, this
has got so much traffic, there's no way there's anything
of significance can be found there. And yet like five
years later you'll find that the find of this injury.
(27:20):
How did that happen? But yeah, and you don't know
what's around in places that you know might be a value.
I mean again, people find I'm still searching for black
Beard's treasure. All kinds of stuff out in time, I
go to the Carolinas out in normal places. Maybe I
should actually go to the everyday beach instead of in London.
(27:41):
That's probably part of the problem. Yeah, you know, Charleston
is so nice. I just like to, you know, spend
some time there enough. Yeah, people even use them for prospecting,
you know, and uh and for coin shooting. Now, if
you do want to use your mail detective for prospecting,
there are some things you need to remember. First, you
have to grow a fear of at least a foot
(28:04):
in length. That needs to be gray and grizzly. He's
gonna punchy ladies and gentlemen. Second, you need to be
able to say things like consarnate with a straight face.
And then third, the most important thing about being a
prospector is you have to develop your crazy prospect or
dance when you strike gold. Very important. Yeah. Coin shooting
is uh the practice of looking for coins after an
(28:25):
outdoor event like a wedding or or like stock you
go to you go to some big outdoor conference and
you take a male detector and you start looking for
for change. The thing is that if you live in
an area where this sort of stuff goes on in
a regular basis, you could presumably pay off that mel
detector after you know, a few visits to these these
(28:47):
areas once the concert is over or a game or whatever. Um,
you know. And and because we're usually talking about change,
you know, some some nickels or dimes or quarters or whatever,
but whatever money you use, because you know, it's not
all just United States folks. Well, if you find euros,
you're rich, because I think a euro is equivalent to
(29:10):
like seventeen dollars at this point. Um, that's a gross exaggeration,
but the point being that, you know, yeah, you could
conceivably pay off your mail detectors costs after a few visits,
you know. Um. But although a lot of people just
enjoy the this as a hobby, like it's just it's
(29:30):
a way to get outside. You're enjoying your joying time
outdoors and you're finding stuff which just brings you a
level of of of satisfaction, right. It means that whole
treasure hunting aspect, you know, when I when we discover something,
there's an element of satisfaction there that you can't put
a monetary value on. Yeah, yeah, you're just like, hey,
(29:51):
look I found some money woo, And yeah, there's there's
an enjoyment level there that goes beyond. Oh and it's
worth you know, seventy five cents. Yes, um, although the cents,
it's always nice to have seventy five cents more than
you had, but when you showed up at the field.
So yeah, I keep wondering, actually, why the value of
(30:12):
the giant jar of class rings. You'd figure they'd cash
that in. Yeah, I mean for more than just selling
metal detectors. Maybe that's more lucrative than selling off some
of those classroom you have, prying out all the little
jewels and melting that stuff down and then just hey,
you know, how can yourself bar a gold? It's worth
its weight in gold. Anyway, I need to go out
and get a mel detector. Yeah they're fun, Yeah they
(30:35):
could be. They could be a lot of fun. Yeah, definitely.
So anyway, that that kind of sums up this podcast
on how mel detectors work. Remember, we do have an
article on the website that goes into more detail and
kind of walks you through what we've talked about here.
I highly recommend it. If you guys have any technology
you would want us to talk about, please let us know.
(30:55):
You can drop us a line on Facebook or Twitter,
are handled there as tech Stuff hs W, or send
us an email. That address is tech stuff at how
stuff works dot com. Chris and I will talk to
you again beep really soon. Be sure to check out
our new video podcast, Stuff from the Future. Join how
Stuff Work staff as we explore the most promising and
(31:17):
perplexing possibilities us tomorrow. The How Stuff Works iPhone app
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