Dot Physics

Could I build a lightning detector?

A while ago, we had a ton of lightning. As a bonus, it always happened in the middle of the night. I love sleeping where it sounds like I am on the front line in WWI – no, I don’t. But, while lying awake waiting for the next BOOM I thought of something. Instead of just counting the time between flash and boom, maybe I could make a lightning detector.

For mere mortals, the first step might be to google “lightning detector”. I don’t want to do that. What fun would that be? I did look up something. The current in a typical lightning strike is on the order of 30,000 amps. How could I detect this.

I can think of two things. Could I detect the magnetic field from this current more than 10 miles away? Or maybe I could detect the change in magnetic flux through a loop of wire. For the second method it depends on the time rate of change of the magnetic field and the electric potential it would generate around a loop.

Modeling Lightning

Let me pretend that lighting is a current that flows in a straight line (a huge wire in the sky). A single current creates magnetic fields that makes a circular vector field around the current. Here is a picture. (I will just use this lovely picture from wikipedia)

i-c5dfee5c23b6a5e539940f7000f8786e-2010-09-01_untitled.jpg

The magnitude of this magnetic field (for a long straight wire) changes with distance from the wire as:

i-c33f1168dd191e30b6629e79055ec40f-2010-09-01_la_te_xi_t_1.jpg

Here μ0 is a constant and r is the distance from the “wire” to the location of interest. So, if I am 10 miles from a lightning strike, what kind of magnetic field could I even detect? This would be a magnetic field of 4 x 10-7 Tesla. Is that a large magnetic field? How about another example. What if I am 10 cm away from a wire with 1 amp of current? That would create a magnetic field of 2 x 10-6 Tesla. So, maybe it would be possible to detect this magnetic field (I will have to play with some detectors to try this).

The other question to address is time. How long does this current last? If it is super quick, I might not be able to even detect the magnetic field. This is directly tied to the other detection method – detecting an electric potential that goes with a changing magnetic field. I am not going to go into all the details (for now), but if you have a changing magnetic field and a loop of wire then the change in potential around that loop would be:

i-cf1e76c9eb075aef9c10294a82120243-2010-09-01_la_te_xi_t_1_1.jpg

Here:

  • N is the number of turns in the coil that makes up the loop
  • B is the magnetic field vector
  • n-hat is the unit vector that shows the orientation of the loop – it points perpendicular to the area of the loop
  • A is the area of the loop

Suppose the area does not change. Also suppose that the magnetic field makes an angle θ with respect to the n-hat vector. This means that:

i-b6dd9fbecce2514a9bee3b91b689b0f8-2010-09-01_la_te_xi_t_1_2.jpg

I honestly have no idea how long this lightning takes to happen. Also, this would give me the average change in electric potential – there are could be a spike depending on how quickly the magnetic field changes. Let me just suppose that Δt is around 0.1 seconds (just a guess) and my loop is a circle with a radius of 10 cm with 100 loops. What would the change in potential in the loop due to the lightning strike be? (assume best orientation of coil to lightning)

i-87a5e6fc14adbe87a151ef311f8a39a3-2010-09-02_la_te_xi_t_1_3.jpg

Ok, that is pretty low. Granted, my value for the change in time might be way off. But if it is not, even increasing the area and the number of loops both by a factor of 10 would still give a potential of around 10-3 volts.

There is another way to detect lightning – by the electromagnetic signal given off during the strike. If you google for “build a lightning detector”, you will find lots of info on these. Ok, but there is a problem. These all detect lightning, but I want to know where the lightning happens. I guess you could build two EM style lightning. If I want to know WHERE, I will need two detectors no matter which way I detect the lightning.

Enough rambling. I guess I should set up some small scale experiments.

Comments

  1. #1 Dave
    September 2, 2010

    Your delta-t is way too large. Use a smaller delta-t. Consider the time it takes for the electrons to make their way from the cloud to ground (or vice versa).

    Consider using a ferrite bar to concentrate the magnetic lines of flux.

    Also, consider using a Hall Effect magnetic field sensor. Allegro makes some nice ones (I like their A1321 device.).

    1 mV is a very strong signal for RF purposes. You’ll need to amplify it, but radio receivers are designed to work down into the uV range.

    Another option may be to use optical sensors to detect the flash. These may be simpler to use than RF or magnetic field sensors.

    Two sensors will allow you to use the timing difference to determine the azimuth to the strike (Note, though, that lightning bolts are not a point source, but are usually a 3 dimensional line source.). To get the distance, you’ll have to add more sensors to allow you to get two azimuths so that you can cross the azimuth lines to determine the distance. Or, use another technique, such as the flash-sound technique, or use radar.

    The real trick, though, is that most people want to determine when/where a bolt will strike at some point in the future so that they can take precautions. For this, look up “field mill”:

    http://en.wikipedia.org/wiki/Field_mill

    Dave

  2. #2 Jim
    September 2, 2010

    I’d bet you already have a pretty efficient lightning detector, otherwise known as an AM radio…

  3. #3 Rob (no, the other Rob)
    September 2, 2010

    My airplane has an instrument made by BF Goodrich (who bought the company “Ryan” who originated the instrument – I bring this up since that happens to be my last name) and now by L-3 Communications called a “Stormscope.” Mine is the WX-10 model. It detects lightning, estimates distance, and plots it on an azimuthal display in the cockpit as either a supplement to or substitute for (in my case) weather radar (which detects precipitation with actual transmitted radar signals).

    The device works by directly detecting the electric and magnetic fields generated by lightning strokes. There’s a bunch of good data on both this instrument, prior art, and lightning in general in Paul Ryan’s patent application at http://www.google.com/patents?id=3Tw7AAAAEBAJ&printsec=abstract&zoom=4&source=gbs_overview_r&cad=0#v=onepage&q&f=false

    It has a range out to 200 nautical miles and I can zoom in to where 25 nautical miles is full screen. It has been, in my experience, very accurate in depicting areas of significant thunderstorm activity (a thing to be avoided in any airplane, let alone a light plane such as mine). You might wonder why I can’t just look out the window. Well, if I’m not in clouds, that’s certainly very effective but in instrument conditions, not so much.

  4. #4 Rhett Allain
    September 2, 2010

    @Rob,

    That is awesome – thanks.

  5. #5 GerwingR Vancouver Canada
    September 3, 2010

    RA you are talking about physics as fun:
    Of course this is. “i have listened to Whistlers”
    i built some multi turn Goniometers in Aluminum tubing. The ELF was an interesting place “and still is” “lets capture and store those 30,000 amp strikes” tongue in cheek, as it is a whole different story. “thumbs up” for the illustration. “lets keep the radio airwaves clean” /rtg/ yvr.ca

  6. #6 NJ
    September 3, 2010

    Of course the cheapest one is to just stand there with an upraised metal rod in your hand.

    Only works once though…

  7. #7 Ed Bernard
    September 3, 2010

    An analysis based upon induction of a magnetic field is a poor way to start. At distances of miles, the long wire magnetic field formula is very poor. At miles the magnetic phenomena is more like a magnetic dipole, whose strength drops as the inverse cube of the distance from the dipole.

    A much better treatment would be based upon electro magentic radiation.

  8. #8 Rhett Allain
    September 4, 2010

    @Ed,

    Very good point – if I am that far away, the lightning is clearly not a long infinite wire.

  9. #9 ThierryD
    October 12, 2010

    You can see it a lightning detector for DSLR camera :
    http://rienquepourlesyeux.free.fr/Detecteur%20de%20foudre/Lightning%20detector.htm
    It work very fine. It is for info.

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