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.
As you can see, the ends of the two headphones lay prostrate next to the two wires connected into the 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):
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.