A Faraday Cage is basically a volume of space that (some form of electromagnetic wave) can't get into. It's like being in a room that's got such well-fitting doors that all outside light is blocked.

A Faraday Cage is like that, but for something other than light. Such as, say, radio waves. Or cell-phone signals.

I propose a special dull-gray paint that would, when painted on a surface, block all cell-phone signals that hit it. Two coats of this on the inside of a room (including the door and ceiling), and cellphones won't work in this area.

Alternately, it could be calibrated for other signals. For example, it could be used to protect the electronics of an airplane from all those high-tech gadgets the passengers have in the passenger compartment (why the airlines insist on that 15-minute shutdown on takeoff and landing).
-- HalfBaker, Oct 04 2007

Probably about as practical as my idea Solar_20Panel_20in_20a_20can
[normzone, Oct 04 2007]

Dunno if it's just a concept... http://gizmodo.com/...ng-paint-157991.php
...or actually exists yet. [neutrinos_shadow, Oct 04 2007]

Same basic idea: Wi-Fi Blocking paint http://www.webmonke...Wifi_Blocking_Paint
[MisterQED, Jun 16 2009]

Microwave ovens discussion http://www.natscien...unded-Or-Ungrounded
To ground or not to ground... [Ling, May 01 2010]

Why a microwave doesn't always block mobile signals http://www.sgurr.co...am/faradaycage.html
The door is not perfectly connected to the body [Ling, May 01 2010]

I bet magnetic paint would do this. You can pick it up at the hardware store.
-- Galbinus_Caeli, Oct 04 2007


This could work. A faraday cage is just a metal box; if you can get good conductivity from the walls to earth then all of the energy of the radio waves is converted to electricity and dumped. If the accepted standard is the copper cage then I'd guess it works better, but you'd get some signal blocking from this, which might or might not be enough for your purposes depending on the strength of the signal and the sensitivity of your equipment.

To post a counter-argument to [21Q], the steam engine has been thought of and rejected too, but that doesn't mean it doesn't work, or wouldn't be appropriate in certain niches... a biomass-based economy, perhaps.
-- david_scothern, Oct 05 2007


Both silver and nickel-loaded paints are used for just this purpose.
-- AbsintheWithoutLeave, Oct 05 2007


The major problem is the required seal tighness. At the highest cell phone frequencies (~2.2 GHz) the wavelength is 13.6 cm. EM radiation can penetrate this well enough that you can't have any openings longer than 1.4 cm, and this is linear openings, not diamater. Thus that tiny little crack along the door is going to let a fair amount of RF radiation through.

There may also be skin depth issues, as some frequencies will pass through to thin a layer, even if it is properly grounded, but I am less familiar with that.
-- MechE, Mar 25 2008


Cool idea, but as for the airplane thing, making passengers shut off their devices is entirely unnecessary for all modern airplanes because they are shielded well enough anyway (as proven by the Mythbusters), but they do so anyway as a precaution, just in case theres a "perfect storm", a unaccounted-for combination of plane electronics and consumer devices that causes failure.
-- Spacecoyote, Mar 26 2008


The bits of conductive material in the paint would have to be larger than the wavelength you're trying to stop. In the case of cell phones - as large as an inch. Not exactly the fine granules you find in your typical paint. Alternatively, you could use some liquid metal - say a mercury amalgam (think silver teeth fillings). Of course that would be hugely expensive, and would give off some rather nasty fumes. Still, I like the idea [+]
-- imho, Jun 16 2009


Very baked, there are several companies who do this as anti-corporate spying steps. I saw it on a show about Bluetooth security on phones. If you want to do it yourself you can try copper based anti-fowling paint for boats. The paint weighs 30+lbs per gallon so I'm sure it will stop all radio waves.
-- MisterQED, Jun 16 2009


Anti-fowling paint? I wasn't aware you could repel birds that way.
-- normzone, Jun 16 2009


Good idea, and very well baked. Those paints run at $1000/gal. What you can do is to find a paint with the same conductivity and is much cheaper than silver, which is currently almost impossible.
-- xkuntay, Jun 16 2009


So, presumably, cell phones only work in aircraft because of the windows?
-- Ling, Jun 16 2009


[Ling] One minor little requirement not really mentioned above, a Faraday cage has to be grounded.
-- MechE, Jun 17 2009


That's something I hadn't realised. Can you explain why? For example, my microwave oven is not grounded...
-- Ling, Jun 17 2009


excess microwaves in your oven are converted into heat.
-- WcW, Jun 17 2009


So, the one's that arent excess are sort of bouncing around? Fortunately, not many of them get out of my ungrounded faraday cage, otherwise I am sure I would have noticed.
-- Ling, Jun 17 2009


all microwaves end up as heat sooner or later. with the door closed they all end up as heat inside the microwave somewhere.
-- WcW, Jun 18 2009


Pretty sure your microwave is grounded, most plugs are.

Regardless, if the cage is not grounded, the signal hitting it produces a charge differential on the inside, which produces a charge differential on the outside, which produces a signal. Grounding prevents the second and thus third steps.
-- MechE, Apr 29 2010


//Pretty sure your microwave is grounded, most plugs are.// Only 2 pins... potentially as dangerous as hell, but I'm still alive.

Last time I checked it: a faraday cage doesn't need to be earthed; otherwise I would have microwaves leaking all over the kitchen worktop, and a minor percentage would heat skin alarmingly. Unplug yours; put your cell phone inside, and then try calling it.
-- Ling, Apr 29 2010


//put your cell phone inside, and then try calling it/
Thanks for that - my baked potato now tastes of Nokia.

BTW, plugged or unplugged, the phone worked)
-- AbsintheWithoutLeave, Apr 29 2010


Now try it with the microwave switched on
-- pocmloc, Apr 29 2010


A two pin polarized plug can still be grounded. (You can ground through the neutral, not ideal, but you can).

Regardless, for a cage to block a signal coming from the inside, it has to be grounded. I might have been mistaken in thinking the same was true for an external signal coming in.
-- MechE, Apr 29 2010


[AWOL], that's interesting. It worked with the door shut? The best way to test it is by calling it, since you can't see ther signal with the door closed. The two pin plug here is reversable. There is no way one side is connected to the case. Often, the earth is supplied as a separate green wire, which ought to be grounded.
-- Ling, May 01 2010


[MechE] I'm pretty sure they have to be grounded (earthed) to block incoming signals, too. Topologically, is there even any difference between "inside" and "outside" in this case?

I've worked in room-sized cages (constructed to block incoming signals), and they were grounded. More relevantly, I've constructed ad-hoc cages, and they didn't block incoming signals without grounding.

As to why copper mesh is usual instead of paint, price/conductivity ratio might be part of the reason, but I suspect mechanical integrity is part of it as well. A small breach will let signals in or out. You probably don't want some charlady in the pay of the KGB "accidentally" scratching the paint.

(I've always thought that "tinfoil hat" people ought to carry patch-cords & clip themselves onto plumbing fixtures wherever they go.)
-- mouseposture, May 01 2010


A few links added. I still am of the opinion that grounding is not necessary (for blocking of mobile telephones: an EMP pulse is different), but there are conflicting reports throughout the sites that I explored.

Biscuit tin trial coming up: doesn't have any 1/4 wavelength door attenuators.
-- Ling, May 01 2010



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