A fun electromagnetic induction demo

A couple of days ago, I had a lunchtime-preoccupation with antennas and electromagnetic induction. We observe a neat phenomenon if you put two wires near each other, and Maxwell stated the phenomenon quite eloquently in his treatise on electromagnetism.  Basically, the instant you change the voltage across one of the wires, the wire next to it (but not touching it) responds with a current that quickly goes away.

By wiggling around the voltage in certain patterns, we can find interesting ways to transmit information back and forth between one another, and through induction we can do this without direct physical contact.  These days it’s easy to take for granted the all the internets sent to us via WiFi and forget about how highly developed an area radio is.

But to get taste of the physical communication, not much is required: basically some power sources and wires. A wire can in fact be practically used as an antenna for certain radio frequencies.

A simple way to check out induction with a laptop is to see if you can transmit a song that usually plays from your speaker through the air and back to yourself.  Since the sound card does both audio input and output as well as the analog-to-digital conversion, a lot of the hairy parts are taken care of for us.

To do this, we need to have the sound output (i.e. from your headphone audio jack) go to a wire.  Next to that wire is another wire that goes into the microphone or “line in” audio jack.  Keep in mind, these are just wires–that’s all.  I just forced myself to part with the headphone that came with my music player and another that I got on an airline flight.

Wires out of headphone and microphone jacks

As you can see, the ends of the two headphones lay prostrate next to the two wires connected into the microphone and headphone jacks:

Close up of decapitated headphones microphone and headphone jacks

So, now all you have to do is start up some music and record the input with some audio recording program (I used GarageBand since I’m on a Mac):

Capturing inducted audio

The green bars towards the middle-right wiggle around while recording indicating that the wire is receiving something.  Doing the same thing without the wires in the line in and out yields no green bars wiggling around.

Because there’s so much power loss, the signal is faint, so I digitally amplified it some in the software.  But horray! I rather shittily transmitted sound back tomyself over air via just two wires.  This, I think, is a neat way to illustrate induction.  Listen for yourself. You’ll need to turn up your volume a bit.

4 comments

  1. Erik Gregg · November 22, 2008

    Awesome! I’m surprised it worked, actually. You may want to check those wires for shielding, common in headphone wires. Also, most of those headphone wires have 2 separate conductors in them, sometimes using the shielding as the second one. You should see if you get better results with a pair of bare copper wires, perhaps by stripping the ends off of the ones you used here.

  2. nequitans · November 22, 2008

    Nice thought. I actually stripped the ends of the wires (the black one kind of shows that in the picture above) and experimented a little right before the post. The distance between them does effect the relative gain.

  3. Brad · November 22, 2008

    Coolness! If I’m ever stuck in a MacGyver moment, where I need to escape an evil Communist prison and contact my friends on the outside, I’ll remember this😉 I’ll just need to find a MacBook or something…

  4. Chris · November 22, 2008

    Cool! What is the loss in decibels? If you compare the transmitted and received wave file’s samples, they should correlate quite well (additive gaussian noise should be low and the time delay from the sound card should be no more than a couple milliseconds) — see how much attenuation the received signal experiences for various separation distances. In decibels: 20 log (Vt / Vr) where Vt is the difference between high and low samples in the transmitted waveform and Vr is the same for the received waveform.

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