
Visualising Electromagnetic Fields - lelf
http://lukesturgeon.co.uk/work/visualising-electromagnetic-fields
======
timthorn
Some years ago, an artist-in-residence at Bristol's Physics dept planted
fluorescent tubes under overhead power lines, which then glowed due to the
field and could be played with by visitors like a plasma ball:

[http://www.bris.ac.uk/changingperspectives/projects/field/](http://www.bris.ac.uk/changingperspectives/projects/field/)

~~~
ChuckMcM
In high school it was a fun geeky thing to do and drive out into the desert
where the power lines from Hoover Dam headed over to LA and light up
fluorescent bulbs. There is a story, possibly apocryphal, that was told by one
of the professors at USC of a guy who lived out there and used a few hundred
feet of 12ga wire to build what would essentially be a secondary coil of an
air gap transformer to power his shack.

I found it interesting but not interesting enough to pursue a career in power
engineering :-).

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balnaphone
An explanation of this kind of visualization given in a stanford lecture by
Steve Mann, see 35-45 min into video.

Stanford Seminar: 42 years of Phenomenological AR for Natural/Reality/Direct
User-Interfaces
[https://www.youtube.com/watch?v=6IpBVC-p3iU](https://www.youtube.com/watch?v=6IpBVC-p3iU)

Example in video
[https://youtu.be/6IpBVC-p3iU?t=2436](https://youtu.be/6IpBVC-p3iU?t=2436)

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ghaff
For a basic magnet, there was an old science fair type of experiment where you
laid the magnet on a piece of photo paper, sprinkled iron filings, flashed the
paper and developed. Of course, these days you can just basically shoot a
digital photo of the magnets and filings. Lots of examples online.

~~~
FreeFull
The one misleading thing about iron fillings is that they tend to arrange
themselves into lines, but the actual magnetic field is continuous rather than
made out of discrete lines.

~~~
msla
That is, in a sense, just calculus: Building a curve out of a huge number of
straight segments is immediately comparable to the method of Riemann sums.

[https://en.wikipedia.org/wiki/Riemann_sum](https://en.wikipedia.org/wiki/Riemann_sum)

I think it forms a nice bridge from univariate calculus to visualizing the
vector fields.

~~~
tlarkworthy
It's not that though. The iron filing change the magnetic field making it
denser where the filing are. So they self organize into reenforcing channels
of flux. So it's not a true visualization of the field, because the field is
changed by the measurements.

~~~
noobermin
Useless pedantic then, it is not a "true visualization" of the B field in free
space absent the filings. It is however, an approximate visualization of it.

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zestyping
These are pretty, and I can appreciate them as art. As a visualization,
though, it doesn't seem very satisfying because most of these images are
pictures of the path traced by the person waving the screen, not pictures of
the electromagnetic field. The shape we see is mainly a visualization of
whatever way the person decided to move the screen through space, that only
incidentally happens to be slightly affected by the field.

~~~
saeranv
Not sure why you're being downvoted, it's a good point. There's a couple of
links here that time-lapse the meter across a consistent 2D or 3D voxel space
which gives a better idea of the radiation field, but some are clearly just
driven by an arbitrary path chosen by the photographer.

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mpolichette
I've always wondered about this... if an EM field is composed of photons which
have higher energy than light, how come I've never seen image/video captured
where the device is 'glowing' or something as a photon source? Is there
something I'm missing that prevents us from detecting them similar to the way
we detect visible light?

~~~
whatshisface
> if an EM field is composed of photons which have higher energy than light

That's not right - visible light is sort of a "midrange" photon energy that's
exceeded by ultraviolet, x-rays and gamma rays and preceded by infrared,
microwaves and radio waves. Everyday devices emit at low frequencies, with
energies significantly below that of visible light. The higher energies
(ultraviolet, xray, gamma) absolutely do have images:

\- Ultraviolet
[[https://en.wikipedia.org/wiki/Ultraviolet_photography#/media...](https://en.wikipedia.org/wiki/Ultraviolet_photography#/media/File:UV_Portrait.jpg)]

\- Xray
[https://en.wikipedia.org/wiki/X-ray#/media/File:Radiograf%C3...](https://en.wikipedia.org/wiki/X-ray#/media/File:Radiograf%C3%ADa_pulmones_Francisca_Lorca.cropped.jpg)

\- Gamma
[https://en.wikipedia.org/wiki/Gamma_ray#/media/File:VACIS_Ga...](https://en.wikipedia.org/wiki/Gamma_ray#/media/File:VACIS_Gamma-
ray_Image_with_stowaways.GIF)

Now, the photons with lower energies than visible light form an interesting
case. First off there's infrared, which is fairly familiar:

\- Infrared
[https://en.wikipedia.org/wiki/Infrared_photography#/media/Fi...](https://en.wikipedia.org/wiki/Infrared_photography#/media/File:Tree_example_IR.jpg)

Now, as the photon energies get lower the wavelengths get longer. This
introduces a problem for lower-energy images: when the wavelength is around
the size of the aperture, wave-like things will happen _involving_ the
aperture. This tends to blur the images, meaning that for longer and longer
wavelengths larger and larger cameras are required to achieve the same level
of detail. However, these images still _exist_ \- but they are rarely taken of
everyday things. Note that the imaging you are talking about (to capture the
glowing of everyday electronic devices in low frequencies) would have to
happen in the microwave and radio bands - because that's around the frequency
at which electronics operate. The result is that a reasonably-sized camera
would not be able to take a meaningfully sharp image of a router's "glow."

\- Microwave
[https://en.wikipedia.org/wiki/Microwave_imaging#/media/File:...](https://en.wikipedia.org/wiki/Microwave_imaging#/media/File:3D_image_of_rebars_with_corrosion_produced_using_microwave_imaging..JPG)

\- Radio Waves
[http://www.gb.nrao.edu/epo/PageMill_Resources/galaxy.jpg](http://www.gb.nrao.edu/epo/PageMill_Resources/galaxy.jpg)

The radio wave picture depicts a galaxy. Radio imaging setups are usually very
large, like this one:
[http://mstecker.com/pages/astroVLAa15vlad-2c1.htm](http://mstecker.com/pages/astroVLAa15vlad-2c1.htm)

~~~
saeranv
So does this mean infrared wavelengths are around the lower limit for taking
images with a reasonable size camera? I've played around with the thermal IR
camera attachements for smartphones (i.e [https://www.amazon.com/FLIR-ONE-
Thermal-Imager-iOS/dp/B00VIL...](https://www.amazon.com/FLIR-ONE-Thermal-
Imager-iOS/dp/B00VILVV62)) and they're pretty small.

~~~
whatshisface
The law for diffraction-limited systems is that the blurred spot size
increases linearly with wavelength. (Double the wavelength, all else being
equal, means twice as blurry.) Engineering concerns (for example, how big can
you manufacture a CCD before it becomes prohibitively expensive?) determine
what constitutes an "excessively large camera." Although, because infrared
covers a 1000-fold range between visible and micro, I feel like it would be
safe to say that cellphone-sized cameras might never be made for microwaves.

[0][https://en.wikipedia.org/wiki/Diffraction-
limited_system](https://en.wikipedia.org/wiki/Diffraction-limited_system)

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kyleblarson
This is cool and reminds me of avalanche rescue training as you have to learn
to follow flux lines during searches:
[http://beaconreviews.com/transceivers/rescue_searching_coars...](http://beaconreviews.com/transceivers/rescue_searching_coarse.asp)
edit: better link

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lovemenot
Excuse my question. I'm curious about a basic formula to calculate magnetic
flux at a given distance from an AC conductor at a given power level?

~~~
tensor_rank_0
[http://www.softschools.com/formulas/physics/magnetic_field_f...](http://www.softschools.com/formulas/physics/magnetic_field_formula/343/)

the strength of the magnetic field is equal to the (current times the
permeability of the medium) divided by (the distance times 2 pi).

magnetic flux is the dot product of the magnetic field and the area vector
representing the area through which you want the flux. theres another way to
express flux as a ?derivative of voltage? but I can't find a reference and the
exact relationship escapes me at the moment.

~~~
lovemenot
thanks

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hansjorg
Another take on this (Light painting WiFi by Timo):

[https://vimeo.com/20412632](https://vimeo.com/20412632)

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ttoinou
Hard to believe. What's the secret to see them ?

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piptastic
They have a video you can watch

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saeranv
The video doesn't really explain what they're doing though. Can anyone explain
how they're visualizing the EM fields here?

~~~
mmcconnell1618
It looks like they have a metering device that glows different colors based on
the strength of the field detected. They then appear to be systematically
moving the meter across a grid with time lapse photography. The lights
photographed from different positions are blended together to create a voxel-
like view of the field.

~~~
saeranv
Thanks for explaining that makes sense. What kind of metering device will glow
based on these kinds of EM fields?

