# Using the audio input port as a voltmeter

I'm a physics undergraduate currently studying electromagnetism. I have no experience at all with playing with electronics. I'm trying to see the effect of a changing magnetic field on a closed circuit (the Faraday-Lenz law)

Now, I have collected some old wires, a male-male audio cable, and some magnets taken from a toy for children. I am also using my MacBook Pro (mid-2009). What I built a rudimentary coil, connected the two ends to two of the pins of the audio cable, and connected the cable to the computer via the audio input port. I expected to be able to see some audio input when I would pass the magnet through the coil.

What really happens, in the best case is just nothing, there is no audio input, even when I make the magnet pass inside the coil. However, when I connect just one side of the coil (which in fact is a long wire) to one of the pins of the audio jack (without connecting the other side), the computer records continuously a fuzz. The first question is why does it happen? I can understand that, given that the two pins are not connected, a potential difference between them is detected, but this happens even the two pins were connected before disconnecting one of them. Why does a potential difference instaurate?

Moreover, I suppose that my experiment is a failure because I'm trying to detect potential differences which are orders of magnitude lower that what I can detect. I have really no experience in the field, so I'd like to ask you:

• what are the voltages of a typical audio input? For my computer Apple says nothing.. and if they are much larger than those produced with my coil, how can a small microphone produce them? How come that even when I use my headphone as a microphone it works, but my system does not
• what is the typical value (in Teslas) of the magnetic field produced by a simple toy magnet?

I'm attaching a picture of the "apparatus", so you can see yourselves what I'm speaking about

Thank you very much for any response

As a physics undergraduate you will know that you need a changing magnetic field to create an electric field in the coil. Manual insertion of a magnet will most likely result in a too low $\dfrac{\delta E}{\delta t}$ to create a noticable current. And like Paul says audio inputs have a high pass filter blocking DC and very low frequencies. The signal from the coil will be too low in both amplitude and frequency.