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I think that three quite different things are conflated here.
(1) My old physics text showed the original Faraday experiment as using a fence of grounded vertical wires blocking a STATIC electric field.
These wires were not cross connected (eg not a mesh or a sheet). They functioned by interrupting the static lines of force (shunting them to ground). The screen must be earthed to function. It is important to understand that the fence of wires is transparent to Magnetic fields and Alternating Electric fields.
(2) A mesh of wires or a solid sheet can be used to shield for AC and RF fields (it acts as a "shorted turn" and shorts the AC electric field). Crucially, it doesn't have to be earthed.
(3) Likewise an Iron or mu-metal shield can be used to shield for Static or AC magnetic fields. It operates by shunting the magnetic field. Likewise it doesn't have to be earthed.
Some practical examples:
The screen between the Primary and Secondary in a modern transformer is acting as an electrostatic shield. It is often a sheet of copper, but can also be a single layer of insulated wire. It must be connected to earth to function, and must not form a full (shorted) turn. It is acting in mode (1).
A modern Screened Room is intended to block AC electrical and RF fields. Strictly speaking it doesn't have to be earthed to function (although it usually is). It is acting in mode (2).
The magnetic shield around the driver in a PC loudspeaker is intended to stop the magnetic field affecting the computer monitor. It doesn't need to be earthed. It is acting in mode (3).
One further example, an earthed ferrous sheet would be operating in all three modes. It is earthing the static electric field. It is shorting the AC electric field and it is shunting the magnetic field.
There are of course many combinations of the above modes.
In my mind item (1) is a Faraday screen. Items (2) and (3) are simply Electric and Magnetic Screens respectively, and the term "Faraday Cage" is a misnomer.
Gutta Percha (talk) 02:45, 6 February 2013 (UTC)[reply]
You have gone way off on tangents here! The terms "Faraday cage" and "Faraday screen" mean exactly the same thing. For things like high-security computers, etc., that need to be protected from electromagnetic waves entering or leaving them (leaking out), they are put into rooms whose walls contain metal screens, similar to chicken wire or window screens built in. Those rooms are called "screen rooms" in the vernacular, and the principle behind the screen room is the Faraday cage. The two things are synonymous. The same principles underlie waveguides, and electromagnetic waves with frequencies too low will not propagate through waveguides. Waveguides are devices in microwave engineering. D.A.W., master of science in electrical engineering.47.215.180.7 (talk) 00:55, 18 April 2018 (UTC)[reply]
Absolutely not! Faraday demonstrated a shield made of a fence of grounded wires. The whole point of his experiment was to show that it blocks electrostatic fields without blocking AC or DC magnetic fields. A Faraday Cage however was a completely different experiment where it was proven that a electric field doesn't extend to the inside of a sphere. It is sad that so many don't understand the fundamental difference between the two. Read any old-style student text where this is made clear.180.150.22.107 (talk) 02:51, 4 February 2019 (UTC)[reply]
It seems there is enormous confusion here because most writers don't understand these three distinct categories.
A Faraday Screen is a fence of separate grounded wires. It blocks only the Electrostatic field. This is what Faraday originally demonstrated.
Example: a layer of grounded wires between the primary and secondary of a transformer, or a fence of wires between the winding of an RF transformer. This is used where a complete sheet would cause a "Shorted Turn".
A Faraday Shield is a plate of conductive metal connected to ground. It blocks Electrostatic fields, and also prevents Electric fields passing through it. Example: an Electrostatic shield embedded in a Transformer.
A Faraday Cage is a completely enclosed box with welded or bolted seams. Example: the classic "Screened room" used in EMC testing. Note that unless the screen is complete, with welded, soldered, or bolted seams it will be almost completely ineffective. It must be constructed from heavy metal (eg Copper sheet or mesh) and must not have wires or pipes passing through the walls (without special treatment).
A possible fourth category is the heavy copper strap soldered around the outside of some Transformers. It's purpose is to prevent radiation of the magnetic field caused by the mains current (eg 50/60 Hz). It operates by placing a complete electrical short around the transformer (eg a shorted turn)
Unless Wikipedia can explain each of these examples which use completely different principles, it will have failed 180.150.22.107 (talk)
Are electromagnetic waves (aka light) blocked by a Faraday cage?
Are electromagnetic waves blocked by a Faraday cage? Electromagnetic waves like the ones used by cell phones and microwave ovens are essentially light in different frequencies. They are different from a electric field. Would loss of reception of cell phone in elevator and microwave oven be right examples of a Faraday cage?
Yes, and no. The difference between light and say cell phone microwaves, as you pointed out, is in the frequencies. Now, the wavelength and frequency have a relationship (in electromagnetic waves), which can be stated as, "wavelength multiplied by frequency is equal to the speed of light" (you can rearrange this equation to solve for wavelength or frequency). OK, so getting to the point: the holes in the Faraday cage are too small for the wavelengths to pass through. So, being that light has much, much smaller wavelengths than say microwaves or radio waves or whatever else might be used for communication, it passes through the Faraday cage easily. In conclusion most forms of electromagnetic waves are blocked by Faraday cages, and I would even venture to say that "all" traditional forms of communication using electromagnetic waves are blocked by a Faraday cage (now as a small point there have been attempts to make a communication device which utilizes lasers to pass the information on, and these would not be blocked by a Faraday cage, but these devices are usually considered impractical because it is necessary for there to be a line of sight for the device to work). Hope this answers your question. Mjs072 (talk) 02:23, 3 April 2008 (UTC)[reply]
There's a paragraph in the "Examples" section that is really bothering me and it's related to the RF vs Light discussion. To whit:
A shopping bag lined with aluminum foil acts as a Faraday cage. It is often used by shoplifters to steal RFID tagged items.[citation needed] In this case, the use of the bags as a Faraday cage is intentional, though paper bags lined with foil are made also for the purposes of blocking light and moisture. (Light blocking by metal foil is actually related to the Faraday shielding effect—it arises from the fact that electrical fields cannot pass through good conductors, because the electrical field across an ideal conductor is always zero, due to its conductivity. Light is electromagnetic radiation just like radio waves, infrared, low-frequency electromagnetic pulses, etc.
What bothers me here is that it implies that anything that blocks light does so because it's a good conductor. Something that is patently false. The initial example of using a foil lined bag to block RFID tags may well be valid, but relating the light blocking properties of a foil bag used for consumer products to this effect is really a stretch.
I would object to removing the whole thing. I agree that a correction is needed, but as you say, the initial example is valid; remove only the offending digression. 67.162.34.172 (talk) 16:26, 4 November 2009 (UTC)[reply]
For instance, will a magnetic compass in a Faraday cage still point toward magetic north? Suggest such knowledge be added to the main page.
Andrew8
No. Magnetic field is not blocked by a Faraday cage. I discovered this the hard way as a student when someone re-located a CRT computer monitor in an adjacent lab. So their computer monitor was 2-3 feet away from the experiment area, but on the opposite side of a wall -- took forever to figure out where the interference was coming from....
Is it just me or is this Faraday cage just a metal enclosure? I suggest someone add to the article that a Faraday cage is actually just a metal enclosure. There seems to be an epidemic in science of easily-reducible-to-a-generalized-name concepts still being called by their original discovers or inventors' last names even one hundred years after they discovered or invented it. I think 30 years of calling a new discovery by the last name of its discoverer is appropriate but not one hundred years plus. Foober22:18, 18 February 2007 (UTC)[reply]
It's more than just a metal enclosure. It depends upon the frequencies that you are considering - for example bridges are Faraday cages at radio frequencies but certainly not at optical frequencies.Flying fish02:26, 6 March 2007 (UTC)[reply]
This is an excellent observation, it made me wonder what types of conductors are non-metallic (graphite, some salts and all plasmas), but i think people often forget they exist (like i did). Although, I have yet to see one made of a non-metal, I suggest that the article should emphasize this, avoiding the use of the word metal unless it is specifing the nature of the conductor as so: ...conducting material (metal or non metal). Chibby0ne (talk) 11:44, 2 July 2010 (UTC)[reply]
It need not be metal but it must be a good conductor. Few materials other than metals are really good conductors. Given economic factors, most practical devices will be metal even though other things might fit the bill in theory. — Preceding unsigned comment added by 67.150.170.103 (talk) 23:12, 27 March 2012 (UTC)[reply]
As others have indicated, it doesn't have to be metal, but it does in some sense need to be an enclosure. The idea of the Faraday cage is more about the shielding effect or restriction of charge to the surface of a conductor. The cage is just the practical realization of the surface of such a conductor. 70.247.169.197 (talk) 03:23, 1 August 2010 (UTC)[reply]
A Faraday Shield consists of a fence of conducting wires or strips which are connected to ground. It blocks Electrostatic fields, but not AC or DC magnetic fields. The key is that it must be Grounded to function. They are wildely used industry where blocking electric fields is necessary, eg coupling between RF coils, transformers, etc. If it were a complete loop it would create a magnetic short circuit. 180.150.22.107 (talk) 02:58, 4 February 2019 (UTC)[reply]
The last part of this article is written in German!
The bit about holes is inaccurate: EM radiation with a wavelength comparable or smaller to the size of the hole gets through easily. That's why the mesh in microwave oven windows has to be so fine. The Anome
I think 'hole' refers to the inside, not to a hole in the mantle. Patrick 14:03 Nov 25, 2002 (UTC)
This whole article has a "subtly wrong" feel to it: it conflates several things as if they are one, such as:
ground plane
Faraday cage principle (exclusion of electric field)
preferential / sacrificial conductor
skin effect
RF shielding
Can people with a physics / EE background please take a look at the article and try to sort this out?
The article refers to a Faraday cage and a Faraday shield as one and the same. I believe when experimenting with capacitors, Faraday discovered that a simple grounded plate could be used to shield electric fields and those are known as Faraday shields. I believe that the cage is a hybrid he discovered later and that is known as the Faraday cage.
Actually he reduced his grounded plate, to a fence of grounded wires. This was the whole point of his experiment and differentiates between a Faraday Screen and a Faraday Cage. — Preceding unsigned comment added by 180.150.22.107 (talk) 03:13, 4 February 2019 (UTC)[reply]
Perhaps we need to break this article into several sections:
Idealised Faraday cage
Mathematics of the Faraday cage
Faraday cage in practice
--> article on Electrostatic shielding and earthing
--> article on RF shielding and earthing
Starting to rework: but it's still wrong... hmmm... The Anome
You need to add a Faraday Screen (a fence of grounded wires). Is fundamentally different to a Faraday shield (grounded plate) or a Farad cage (completely enclosing box with welded seams). — Preceding unsigned comment added by 180.150.22.107 (talk) 03:18, 4 February 2019 (UTC)[reply]
"Since like electrical charges repel each other, they accumulate where they can be furthest apart, on the outside layer of the Faraday cage." ... This is a description of the electrostatic principle that the electric field inside a conductor is zero; it is not applicable to changing EMFs. I'm rewriting the paragraph... --hb
The excited fields oppose the applied fields, in concord with the law of conservation of energy.
The relationship is not clear to me. Patrick 11:11 Nov 26, 2002 (UTC)
a current that cancels the applied field inside the conductor
This may create the impression that as long as the electric field is outside, the current flows to cancel it inside; however, in the case of a static field, the charge distribution (+ here, - there) takes care of it, the current is there only very shortly to create this charge distribution. Patrick 11:56 Nov 26, 2002 (UTC)
I agree that the article seems to include stuff that isn't strictly within the original definition of the Faraday cage effect. I added some historical info at the beginning to show where the idea started. Perhaps with more research we can trace how the concept has been broadened to RF etc., and whether the later additions deserve to be part of this article. -- Heron
The concept doesn't have to be "broadened" to include RF. It includes all electrical energy. In practical F Cages, instead of smooth walls, a mesh is used to save metal/cost (or for visibility), and in that case the wavelength of the electrical energy becomes significant vis a vis the mesh size; obviously, the EM energy at the optical wavelengths are meant to get through. If the cage is completely closed and smooth (e.g., a closed sphere), classical EM theory predicts zero field inside everywhere. Of course, quantum considerations change everything...
There has been some back and forth about FCs on (ironically) the tin-foil hat page. Take a look at the discussion (and the talk page) for some more information about skin effect, partial FCs, etc., that could be usefully integrated.
What exactly is the difference between the concept of a Faraday Cage and the Skin Effect?
They both seem very very simular, except that the skin effect is based on alternating current.
I have read that the real reason why a person is safe in a car during lightning is the skin effect, as a lighning bolt acts as alternating current, although only one half wave of it.
The first paragraph seems to contradict itself, defining a Faraday cage as shielding from electromagnetic radiation and then talking about Michael Faraday experimenting with electrostatics. Obviously the cage will shield both, but this does not make clear what the definition of the term is (i.e. perhaps it was initially an electrostatic shield and then discovered to also be an electromagnetic shield). Either way, as mentioned above, the article could probably do with a rewrite by someone with a background in the area. --postglock08:36, 18 July 2005 (UTC)[reply]
I hope interested readers can discern that the Postglock's post was addressed shortly after his post. The article is correct. See also skin effect for the RF (i.e. electromagnetic version) --Ancheta Wis00:01, 18 December 2005 (UTC)[reply]
The article is wrong. Faraday used his Faraday Screen to differentiate between Electrostatic and Electric fields (eg AC v/s DC). Faraday Screen and a Faraday Cage are two completely different things. 180.150.22.107 (talk) 03:10, 4 February 2019 (UTC)[reply]
Needs more details on the differences between magnetic, electric, and electromagnetic fields, frequency, near field vs far field, and which of those a Faraday cage shields against. and real-life, resistive cages vs ideal perfect conductors. — Omegatron03:22, 22 December 2005 (UTC)[reply]
Is this description correct?
Magnetostatic fields would not be blocked by such a shield, but changing magnetic fields would create eddy currents in the surface that create their own fields and cancel them out, so the magnetic shielding gets more effective at higher frequencies?
Changing electric fields would not be blocked as much as electrostatic (which are blocked perfectly), since the charges have to move around to cancel them out, and charges can't move instantaneously, so the shield would be less effective against electric fields as frequency increases? — Omegatron16:10, 23 March 2006 (UTC)[reply]
NO! ANY time that you have a time-varying magnetic field, you automatically have a time-varying electric field. This is a basic law of the Universe. Please do not argue with Faraday's Law and Maxwell's Equations. Doing otherwise makes you look ignorant!
Faraday cages do not block DC or slowly-changing magnetic fields. A magnetic compass will work inside a Faraday cage.
From <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations">Maxwell's Equations</a>, however, a changing magnetic field creates an electric field (and vice versa), and the walls suppress the electric field. They therefore also suppress the magnetic field. Whether the changes are "fast" (and therefore suppressed) or "slow" (and transmitted) is set by the wall materials' thickness and <a href="http://en.wikipedia.org/wiki/Skin_depth">skin depth</a>.
Faraday cages large enough to people, furniture, etc. are often called "screen rooms".
They're also used in pulsed high-voltage experiments to protect sensitive electronics from electromagnetic pulses created by the experiment.
It seems that a Faraday Cage requires a completely enclosed cage so that there are clear "inner" and "outer" surfaces. However, one of the links on the bottom goes to a site that talks about grounding a conductive shield and specifically says that the Faraday Cage does NOT have to closed on all sides. It seems to me that if the cage is NOT closed on all sides, then the conductor has only a single surface and the electrical charge is free to propogate to any point on that surface. (The linked site also credits scientists working with Einstein or at least in the same era for creating the first Faraday Cage, but I digress.)
So is this a different property than Faraday's enclosure? Is this more simply, shunting the electrical charge away and dissipating it to ground? Does a Faraday Cage need to be grounded? Is a Faraday Cage more effective either for blocking electrical fields or as an RF shield if the cage is grounded? If the cage is not fully closed on all sides, is it a "Faraday Cage"? --JJLatWiki16:08, 8 June 2006 (UTC)[reply]
A Faraday cage does not need to be grounded the shield the inside from external EM. The reason cages are connected to an internal circuit's ground is to prevent the cage from capacitively coupling parts of the circuit to each other, and to prevent crosstalk from cables. Connecting the shield to ground causes them to capacitively couple to ground instead. There's a very clear description and picture in ISBN0-7506-7403-2, which we should mimic. — Omegatron06:26, 25 November 2006 (UTC)[reply]
From the current article "If the cage is grounded, the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage would remain neutral."
Is this not a mistake? I would think that the charge needed to nullify the field created by the internal charge (inside the conductor) would be supplied by the ground - not the other way around (as in the above citation) --Qff (talk) 09:33, 29 August 2010 (UTC)[reply]
It is an unclear and potentially (aha!) misleading sentence, and adds to the confusion in everyday speech between a "ground" (as in V=0) and something "neutrally charged" (as in Q=0). The ground is a reference potential, not necessarily without net charge, and I believe it is a mistake to start talking about charges moving back and forth here, since the ground potential is undefined. 83.248.163.42 (talk) — Preceding undated comment added 08:10, 16 December 2014 (UTC)[reply]
Added to the list item indicating occupants in a car were safe from lightning. The truth is more conditional
than that and cars don't provide complete safety.
Question: What does this comment "Faraday cages are not good to use outside in the event of a lightning strike." mean? The cited source does not support the assertion. Perhaps it can be reworded. --69.3.237.16921:03, 24 November 2006 (UTC)[reply]
Is protection from electrical currents really an aspect of a Faraday cage? I mean, I know that Faraday cages protect against currents, but is that really relevant? The key point of Faraday cages is that they prevent electric fields from penetrating. Protection from currents is related more to the skin effect, no? — Omegatron07:41, 25 November 2006 (UTC)[reply]
The intro says that Faraday cages keep out "electromagnetic fields". Do they indeed keep out magnetic fields? Naively, I would assume that they keep out electric fields, and therefore electromagnetic radiation, but not necessarily magnetic fields (unless the conductor happens to be magnetizable, but that's not part of the definition of a Faraday cage).
The intro gives as core reason for the operation of a Faraday cage that repelling charges will accumulate on the surface of a conductor. While that is true, it doesn't explain how a Faraday cage blocks an electric field. The explanation further down in the article (moving charges creating a field that cancels the original one inside the cage) is much more to the point. I would argue to remove the intuitive explanation in the intro, or replace it with a better one.
I'm also interested in the relationship between the mesh size of a Faraday cage and the wave length of the radiation that it blocks. Does the mesh size have to be significantly smaller than the wave length? My little hand-held radio can receive radiation with wave length of over 3m just fine in a car (whose mesh size as a Faraday cage is smaller than one meter). AxelBoldt21:40, 19 September 2006 (UTC)[reply]
I don't understand this either, and I should. From what I understand, a Faraday cage keeps out static electric fields but not static magnetic fields. It also keeps out changing electric fields, though not as well, and worse at higher frequencies, due to resistance of the material, and also blocks changing magnetic fields, better at higher frequencies, due to induced eddy currents. And those are all near field. Far field EM I don't know about, and I'm sketchy on the difference between near and far field anyway.
As for your radio working inside the car, blame it on the tires. Rubber tires. Unless you're dragging a grounding strip, your car body is very very poorly grounded. Ungrounded Faraday cages are only practically effective against static EM. [Completely FALSE]
While it is technically blocking the signal arriving from the radio transmitter, in your case, all that RF energy goes into inducing currents in the cage, just like any other antenna, and since there's no ground for that current to dissipate into, enough of it ends up re-emitting into the inside of the cage for your radio to still get decent reception. You'll weaken the signal nonnegligibly, and introduce a lot of extra RF noise, but standing between two skyscrapers does that. Your radio has circuits to filter out the noise, and the signal coming in is apparently strong enough to afford the energy loss. 67.162.34.172 (talk) 16:58, 4 November 2009 (UTC)[reply]
At low frequencies, even absorption shielding is useless.
For low frequencies, use high-permeability shielding to re-route the magnetic field lines, like mu-metal
Summary of facts from ISBN0-521-37095-7:
Low frequency magnetic fields are not shielded well by metal enclosures. Instead, try not to create loops in the circuit layout, and if you do, don't let them cover a large area. Use twisted pairs to connect things, since they minimize loop area and each successive twist picks up the opposite field of the last, canceling out. Can also use mu-metal shielding. — Omegatron07:11, 25 November 2006 (UTC)[reply]
I can't argue with any of that.
Static magnetic fields are not shielded at all by a Faraday cage.
Far fields are those that have an E and an M component at right-angles to each other and to the direction of propagation, like EM waves in space. The wave impedance of free space is 377 ohms, while that of a conductor is low (milliohms, and a complex number [1]). If you short out the E component with a conductor, the M component gets clobbered too, since the two components generate each other. In near fields the two components aren't related in a simple way, so you have to consider them separately.
The attenuation of a hole is proportional to the ratio of wavelength to diameter. There isn't a sharp cutoff. This source [2] (table on p2) shows that at half a wavelength you get -3.0 dB, so you shouldn't be surprised to get some leakage.
"For circular apertures which have diameter much smaller than a wavelength, the radiation coupled through is proportional to the sixth power of the radius." [3] (right at the end), hence the microwave oven mesh. This seems to contradict the previous point, so I suppose the two statements apply to different ranges of λ/d.
A thought experiment. If the cage were superconducting, it would keep out very LF magnetic fields, and even switching on an external DC electromagnet would count as changing the magnetic field, so the eddy currents induced in the cage would circulate for ever, cancelling out the field. --Heron17:05, 26 May 2007 (UTC)[reply]
I've heard that some recording facilities use a Faraday Cage. Can anyone verify this or provide more information? MikebrittUTC
Most early recording gear was badly designed from an EMC point of view. This means that it suffered from mains-born "clicks and pops" as well as radio break-through (mobile phones and such), so rather than redesign the gear it is easier to screen the whole studio.
The studio that Wendy Carlos built in her New York City brownstone is fully enclosed in a Faraday cage. From her web site: "...the new studio, in being a genuine Faraday Cage (conductive walls, ceiling and floor, tied to common ground) is truly free from essentially all external signal contaminants."K8 fan (talk) 17:05, 3 June 2013 (UTC)[reply]
Which rather implies that a "genuine" Faraday cage must be grounded.
Which is, in fact, what I was taught: that a true Faraday cage is an enclosure made of electrically conductive material that is tied to a common ground point. — Preceding unsigned comment added by 74.95.43.249 (talk) 20:48, 6 October 2015 (UTC)[reply]
No. A Faraday Screen needs to be grounded, but a Faraday Cage does not. However, unless a Faraday Cage completely encloses the DUT (with welded seams, etc), it will be ineffective. 180.150.22.107 (talk) 03:08, 4 February 2019 (UTC)[reply]
A bolt of lightning has over a billion volts and a lot of amps, but if it hits a car, it won't penetrate through it, and kill the people inside? 64.236.245.24314:35, 1 March 2007 (UTC)[reply]
This was tested on the BBC's Top Gear. Richard Hammond sat in a car while an electrical charge was built up in a device high above it until the car was effectively struck by lightning several times. The electricity did penetrate the car a little since he reported feeling it in his fingers, but not enough to do any harm. 194.80.52.106 (talk) 14:12, 22 August 2008 (UTC)[reply]
The article mentions that small electronic parts at Fry's are kept in a cage, inferring that the cage is used as a Faraday shield. What I've seen at any Fry's, I've ever been to, is nothing but a security cage made of chain-link fence. There would be no reason to keep parts in a Faraday cage; there would however be a reason to keep them in a secure area; CPU's and memory are valuable and easily stolen.
24.165.87.7603:01, 14 May 2007 (UTC)[reply]
Note that the suits worn by high-voltage powerline workers are not meant to protect them from the current that flows in the line, but rather, from the intense electric field that surrounds it. Without the suit, the AC electric field would induce circulating electric currents within the lineman's body. 74.111.99.101 (talk) 20:12, 15 March 2021 (UTC)[reply]
My thoughts: To get a "Faraday's cage effect" it can have big holes and must not be metal as long as it has low resistance (metal helps both because of that the charges inside the metal extremely quickly re-distributes which in total makes the field zero inside the cage, but also might help because of the low resistance since you get a voltage division vs the source), and it certainly does not have to be grounded. See my comments on "Antistatic bag" too. --88.131.22.138 (talk) 14:59, 11 March 2008 (UTC)[reply]
I agree -- grounding a Faraday cage, or not, is completely irrelevant to its operation. That is total science fiction. For example, an aluminum-alloy airliner in flight is a Faraday cage, and an ungrounded car is a Faraday cage. For Faraday cages in buildings, grounding them solidly IS a good idea for safety's sake in case of electrical accidents within or lightning strikes from without. When lighting strikes a car, the (insulating) rubber tires are bypassed by the current jumping the short gaps between the underbelly of the car (incl. the hubs) to the ground or pavement.47.215.180.7 (talk) 02:29, 18 April 2018 (UTC)[reply]
At a business that was located across a two-lane street from one cell providers antenna, several employees including myself did an experiment on our lunch break that involved sticking the phone inside of the microwave, with it powered off the whole time, and seeing how many bars remained and/or calling the phones. Most phones went to out of service. Two rang, including the phone that matched the service provider on the antenna farm across the street. The matching-provider phone kept 100% of its bars during the entire time it was inside the microwave.
The cellular band (1.8, 1.9GHz) isn't too far from the 2.5GHz used to actually cook the food. It does a pretty mediocre job of keeping these larger wavelengths out, but yes it did in fact block them somewhat. Just not as well as would be expected if the device were designed to do such, considering how frequently the signal is blocked unintentionally.
It seems like we should be able to get a citation on exactly how much of a shield a microwave oven is, or at least ought to be, considering that we are allowed to have 1W WiFi amplifiers on bookshelves and these ovens are usually between 400 and 1200 watts; I would think that somewhere, there ought to be a published guideline for shield effectiveness. —Preceding unsigned comment added by Zaphraud (talk • contribs) 20:27, 9 May 2008 (UTC)[reply]
Bold textThe theory is seductive, 'blocks radio since the size of the holes are smaller..'. Now the practice. A fm radio in a microwave cabinet is a lot attenuated. But the much longer mediumwave wavelength only 10 dB's ! Grounded or not made no difference. Now why should it block microwaves 50 dB and much longer waves only 10 dB's ? And light is only a radio frequency ?
Well this can be shielded by thick paper and wood. The neighboring frequencies are passed. Why?
Something very strange is going on here.Bold text — Preceding unsigned comment added by 84.104.74.12 (talk) 10:20, 10 April 2016 (UTC)[reply]
While I'm not sure how much attenuation is needed to block a cell phone signal, I do know that I cannot get service inside a closed shipping container.
Microwave ovens leak about 1mW/cm2, which is only about 40-50 db less than what is inside the microwave. —Preceding unsigned comment added by 24.74.206.8 (talk) 05:21, 18 December 2010 (UTC)[reply]
The phrase "A Faraday cage is best understood as an approximation to an ideal hollow conductor" really puts me off. I hardly know anything about how Faraday cages work, but this is a very bad way to start a section to explain how something works. I myself don't understand them so I don't think I'll change this myself. If anyone who knows more about Faraday cages could fix this it would just be great --BiT (talk) 06:51, 14 January 2009 (UTC)[reply]
metal frame and outer skin of the vehicle cause the electrical charge
to travel safely away from the occupants. This differs from a popular
urban legend that claims that a car's tires cause the lightning strike
to reach the ground. However, radio and cellular phone signals can
still reach inside the vehicle since their wavelengths are
significantly smaller than the windows and other openings in the
vehicle's conductive frame, though internal signal strength may be
diminished.
Kindly mention exactly how long the wavelength of lightning is.
SORRY. This statement is INCORRECT. AC or DC is actually irrelevant. An electrical current/signal does NOT have to change direction (AC) to have a Wavelength or frequency, it can be DC and still carry a varying waveform. It only has to change in strength ie. voltage over time. In many (most?) applications ie oscillators a signal will change direction.
It is usually depicted, as here in the Wavelength article, changing direction/polarity. BUT it does NOT have to reverse direction to have a wavelength. It could change from +5 to -5 Volts, or 0 to +10 Volts. As shown here, the measurement between 2 'equivalent' points on the waveform is the same irrespective of polarity. Are you perhaps thinking of DC in respect to a battery?
Yes, Lightning is a huge pulse of DC. As DC it has no wavelength, however the Transient has a very wide range of AC components and hence the range of Wavelengths is effectively infinite. — Preceding unsigned comment added by 180.150.22.107 (talk) 03:23, 4 February 2019 (UTC)[reply]
If they're wet enough, they can make a passable Faraday cage, and yes, an outstandingly well-grounded one. However, at this point, any Faraday-cage-related, charge-distribution-based, shielding effects are totally insignificant compared to the sheer mind-boggling quantity of stone and dirt and hard water and other not-perfectly-transparent-at-useful-frequencies materials between the inside of the cave and the surface world, or even, in many cases, between rooms of the same cave.67.162.34.172 (talk) 17:37, 4 November 2009 (UTC)[reply]
I cannot understand why the only example offered on this page relates to a foil lined shopping bag. Surely there are other examples which can be offered to the reader to better understand this concept... 142.221.110.4 (talk) 19:14, 22 September 2009 (UTC)[reply]
Maybe not an example but a good reference in popular culture is the movie "Enemy Of the State". At one point in the movie, the ex-CIA agent steps into a Faraday cage to prevent eavesdropping/location. —Preceding unsigned comment added by 131.247.83.135 (talk) 13:33, 29 October 2009 (UTC)[reply]
That's a good example actually. Faraday cages are used in the electronics industry when 'aligning' RF (Radio Frequency) transmitters( TX) and receivers (RX). Stops the TX which may be off frequency from causing interference, allows RX to be tuned without interference from external TX sources etc. Also stops RF from test equipment causing interference to other external RX and electronic equipment.
Faraday cages also exist inside equipment. ie. inside a mobile/cell phone there is a lot of metal covers. At least some of this is for RF shielding. CRT TV/monitors (probably LCD/Plasma/LED displays too) have metal shields for various parts to stop interference getting out (& in too). This should fit the 'technical' definition of a Faraday Cage. Most often though, in my experience, it is merely referred to as 'shielding'. See Electromagnetic shielding. and try Practical Electromagnetic Shielding Tutorial [4]
The 'code room' in embassies and other places that use secure communications would use a type of Faraday cage to stop eavesdroppoing on communications. (Some of these are like bank vaults anyway!) The equipment itself would also be shielded to stop 'cleartext' signals getting out. ie being accidentally transmitted with the 'encrypted' text. This is a method that HAS been used against cypher equipment with poor 'shielding'. The technical codename (once classified) that 'Security agencies' ie NSA use for this area is TEMPEST.
Apart from books and websites user:Elonka is very much into deciphering encrypted messages. (She was once invited to the CIA to give a presentation!) She might also know a bit about this side of things. --220.101.28.25 (talk) 08:11, 22 November 2009 (UTC)[reply]
I wish I had a citation for the story told to me by a DEC hardware engineer, the gist of which was: one of his cow-orkers was called in to attend a sick VAX at GCHQ in Cheltenham. As can be imagined, many were the hoops requiring to be jumped through before he could gain access to the machine room, which was located inside a Faraday Cage guaranteed to stop the pesky Soviet Union from finding out what was going on inside. Great, then, was the consternation among the spooks when, while up to his elbows in the entrails of the unwell VAX, his pager went off. Mr Larrington (talk) 10:20, 16 August 2016 (UTC)[reply]
From the first sentence of the second paragraph, under History:
The same effect was predicted earlier by Francesco Beccaria (1716–1781) at the University of Turin, a student of Benjamin Franklin, who stated that "all electricity goes up to the free surface of the bodies without diffusing in their interior substance."
I read this as attributing the "all electricity goes..." quote to Beccaria, not Franklin, but the phrasing is ambiguous on the matter. Could someone who knows for sure what it should say, please rephrase this sentence to be clearer? 67.162.34.172 (talk) 17:45, 4 November 2009 (UTC)[reply]
According to my memory of first year physics, a Faraday cage only blocks incoming electromagnetic radiation - to block outgoing radiation would violate Gauss' Law. Considering removing the whole section but I'd like an expert's opinion first. — Preceding unsigned comment added by Akdor 1154 (talk • contribs) 03:46, 8 September 2011 (UTC)[reply]
A Faraday cage WILL block outgoing radiation in the same way as incoming radiation. To block outgoin radiation would not violate Gauss' law since a radiation source inside the cage cannot change the amount of charge inside the cage. The total charge inside a Gauss surface surrounding the cage will not be affected by charge movement inside the cage. This beeing said I tried the experiment with a cellphone in a microwave oven and still got good reception. Ulflund (talk) 07:41, 9 October 2011 (UTC)[reply]
The Operation section of the main article states that the position of a charge inside a hollow metallic conductor does not affect the field generated outside.
Again, this is an oft misquoted form of Gauss's Theory. Yes, the field outside will be identical to as if the conductor wasn't there - but moving the charge within this conductor, and RELATIVE to it, will change the strength of the field at a defined external point. — Preceding unsigned comment added by 123.238.25.145 (talk) 00:52, 26 September 2011 (UTC)[reply]
Gauss's theory is not misquoted. Charges in the metallic conductor will move in such a way as to cancel any movement of the contained charge. This is not something you can see directly from Gauss's Law, but has to do with the free charges in the conductor. Ulflund (talk) 07:47, 9 October 2011 (UTC)[reply]
I think the main article, and several related ones could do with linking to an article on electromagnetic transparency.
Speaking of transparency, I've also got a question
> How are certain substances transparent at certain regions in the electromagnetic spectrum, but not in others - on EITHER SIDE of this permitted region?
E.g. The Human body: X-rays get through, radio waves pass through, but light does not.
The whole point of Faraday's original experiment was that a fence of grounded wires blocks an Electrostatic field, but not a Electric (eg AC) field or a Magnetic field. 180.150.22.107 (talk) 03:28, 4 February 2019 (UTC).[reply]
As a matter of protecting electrical equipment from solar flares or other peaks in EMF, a Faraday cage is often advised. Is such a cage required if the equipment is turned off and the battery removed? Can someone provide a source in regards to that?-65.189.245.24 (talk) 14:00, 4 April 2013 (UTC)[reply]
The other link used in this reference directs the reader to what is apparently an archived copy of the Encarta entry.
That entry mentions a Faraday cage only once - briefly.
Why would an encyclopedia use another, discontinued (according to that article's references, Encarta's discontinuation could be attributed to Wikipedia) encyclopedia as a source? Surely there are better and more reliable sources available, that are not simply agglomerations of information into a handy reference. While Wikipedia's guidance on reliable sources does allow that "Reputable tertiary sources, such as lower-level textbooks, almanacs, and encyclopedias, may be cited", it also implies (in a navel-gazing, self-referential manner) that these are not particularly valuable sources - the information they hold should be available for citation from another source.
Finally, this article is of a length, and contains content sufficient that one assumes additional sources have been used and thus there should be more than the existing three references in an article of (many) words. Ambiguosity (talk) 07:52, 4 June 2016 (UTC)[reply]
Polarization of EM Frequencies, Surge Suppression, Carrier Noise and Clarifications
After review of the factual and speculative interrogatories contained within this wiki article
asking for verification or demonstrating the fundamentals of Faraday Grids, Cages, etc.,as well
as, the rudimentary physics and quantum analytic discussions contributed are flawed, defective
and indefinite as a whole, even though most contributions are pointed in the right direction.
As the Subject headline suggests polarization, etc., are all missing in the content of this wiki
causing ambiguous assumptions, queries of all sorts and literal confusion particularly the effects
experienced by the human 3rd dimensional entities inhabiting the earth planet.
All things, animate or inanimate are seemingly solid in their relationship to its elemental form.
It is therefore reasonably accurate to suppose that all things seen, unseen, measurable or not, is
subsequently reduced to energy vibrations in their relationship to infinite frequencies, harmonics,
chaotic noise generated and mixed in with the root of any waveform pattern, however equated.
Faraday focused on blocking of various electrical energy patterns which interfered with the operation
of his historical day contrivances. Trying to apply Faraday quantum elements to present day applications
is incomplete and therefore antiquated today. However, lays the groundwork for further expansion that
needs to be applied to present day issues considering both the microcosm and macrocosm of pure energy.
For example, a contributor queried light wave spectrum but missed the side bands of light waves, missed
the concept of as speed approaches the speed of light, time becomes zero and further did not express any
considerations to noise accumulated on root frequencies that Faraday was concerned with in his time.
The noise includes side band frequencies outside the mainstream light wave spectrum, such as gamma, theta,
beta from ultra violet to infrared spectrum which piggy back on these frequencies, as common knowledge to
those in the electrical fields have a very clear understanding as we in these fields work with suppression
of transient, white, pink and other noise to clean up, so to speak, the clean generated frequency energy.
Moreover, grounding is the most important element in the electrical physics of energy and even more so
when considering the polarization of macro and microcosmic physics extending beyond the light wave spectrum
and beyond its speed when the portal of time is no longer relative but cyclical in lieu of wave forms in
time and space. The latter being the foundation of Faraday quantum physics and the blocking of these forms.
The principle theory of polarization is the proportional repelling of opposite charged elements, much like
the opposing poles of any set of bar or u style magnets. The more complex side of polarization extends to
both the effects considering the microcosm within ameba structures as well as that in the macrocosm of
planetary bodies and their respective valence bonds. Reference is emphasized to Nikola Tesla experiments.
Subsequently, a new vision to understand sub atomic particles, such as quarks,to electromagnetic fields of
the planetary bodies including all the conglomerate of the joining of noise inter-meshed with the host
of root frequencies will require the rethinking of the engineering for any type of Faraday cage or grid,
that will polarize these more complex conglomerate frequencies which are manifesting in present day EMP's.
In conclusion, Faraday grids or cages, let alone his solid forms will require an advanced infinite concepts
of energized and polarized fields impressed on these type of grids in order to be effective in this current
day and age of digital signals and the transient transmissions of analog finite and cyclic infinite energy.
PS: The caption refers to the mainstream mathematical description of the problem. I didn't dare to further specify the caption due to the common reader's first impression.
My apologies if this is encompassed or implied by a previous answer, but does anyone know whether Faraday cages can be used to protect electronic devices from those EMP weapons that knock out every device within a substantial radius? In any event, the cage's usefulness or non-usefulness for this purpose should be mentioned in the article. Antediluvian67 (talk) 00:20, 13 February 2017 (UTC)[reply]
Yes, a Faraday Cage can block EMP. The problem is that a truly effective Faraday Cage is difficult to make.
It must me made of heavy sheet or mesh (eg thick Copper), with all of the seams welded or tightly bolted, and no wires passing through the walls. And preferably with double walls. If not it will be surprisingly ineffective. This combined with the enormous strength of a true EMP pulse, makes building an effective screen very difficult and expensive. 180.150.22.107 (talk) 03:35, 4 February 2019 (UTC)[reply]
Hi, I recently came across a source which states that Faraday's experiments with regards to the cage effect were similar to earlier experiments performed by Nollet. However, I have been guilty of being too enthusiastic in adding new information to wikipedia and sometimes it results in me adding things that are erroneous or unsupported by references without me realising it so I would like to make sure I'm not being too hasty this time. You can find the link below and based off what little I was able to translate it sounds like Nollet may have also discovered the Faraday cage effect early though I'm unsure of the exact date and whether or not I'm misinterpreting things again which isn't helped by the fact that the source is entirely in French, so I would like some other wikipedians to pass judgement, particularly French ones.
Alright, it's been a week with no response and, having seen a few other sources that strongly suggest this was indeed an early detection of the Faraday cage effect, I have decided that I'm going in.SQMeaner (talk) 08:00, 2 April 2019 (UTC)[reply]
I would like to contest the statement: "A good idea of the effectiveness of a Faraday shield can be obtained from considerations of skin depth.", as it relates to electric fields at least (magnetic fields are another consideration). This statement appears in the section at: https://en.wikipedia.org/wiki/Faraday_cage#Exterior_fields
I think that for varying electromagnetic fields, the majority of the energy is actually reflected at the surface of the conductor of the Faraday shield, thus the attenuation (skin depth) considerations are moot. From section 9.3 in Griffiths' Introduction to electrodymanics (4th ed.) we can calculate the transmission coefficient for any air-metal interface and see that it is extremely small. It is this transmitted electric field that actually gets attenuated as per the skin depth effect, but the majority of the shielding comes from the low transmittance (high reflection), not the skin depth effect.
To convince ourselves of this fact, consider this informal thought experiment: in electrostatics we know that the thickness of the shield is not relevant in order for the shield to work properly: the conductor could be very thin and the Faraday cage would still work perfectly. Now if skin depth alone is used to determine the required thickness for effective shielding from an electromagnetic wave, then lowering the frequency implies that a thicker conductor is needed. In the limiting case where the frequency is extremely small and both concepts should reconcile (0.0001 Hz for example), the electrostatics view would mean that a thin shield can be used, while the skin depth view would imply that a very thick shield is needed (which is a contradiction). From experiments we know which view is correct and we know that a relatively thin cage will still work effectively at low frequencies. We can only conclude that the skin depth is not the main factor in determining the required thickness, and thus the original sentence in the article (and the accompanying graph right beside it) are misleading.
To further intuitively appreciate the impact of low transmittance and high reflexion at an air-metal barrier, the principle of radar and how it easily detects metal objects can be used in place of any calculations alluded to in the Griffiths reference above :-) 142.137.139.167 (talk) 14:46, 21 September 2022 (UTC)[reply]
Whoever added the video definitely had the right idea, but it is quite low resolution. Surely it would be worthwhile to replace the video, or to put it through some kind of AI enhancing tool? If you all are cool with me replacing the video, could you please let me know? Data Devourer (talk) 05:13, 8 January 2024 (UTC)[reply]
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