The contemporary world produces a sea of electromagnetic (radio) noise. Most electronic devices emit radio byproducts as they operate – especially motors and microprocessors. A small fraction of these emissions occur at frequencies that we could hear if we could sense electromagnetism… but luckily we can’t.
A coil of wire wrapped around an iron bar can detect these fields – and allow us to hear them. It will produce a voltage proportional to the fluctuating electromagnetic fields that cross it, via electromagnetic induction. This is nothing new (it’s the foundation behind transformers, motors, electric guitars and tons of other common devices) but nonetheless it’s fascinating to experience this hidden world with your ears.
Back in the days of Ma Bell, the inductive “telephone pickup” was a common way to record a phone conversation. It picked up the electromagnetic field produced by the handset’s speaker, without making any electrical connection to the phone system. (Thus undetectable to the other party.) Radio Shack used to sell them, but good luck finding one these days. Tinkersphere in NYC sells them, or spring for the super duper stereo boutique option, the LOM Elektrosluch!
New Life For Old Relays
Recently I almost threw out some un-needed 120V relays, but decided to use them as inductive pickups instead. I extracted their coils and soldered them onto an 1/8″ stereo cable. The fragile coil wires are anchored with hot-glue, and large heat-shrink tubing protects against “fall-apart”.
Some short examples:
Minidisc Recorder (spinning up the disc a few times):
Two mobile phones:
The inductive pickup only works when held close to the source, so it’s able to pick out individual fields in the crowded electromagnetic atmosphere. If you’re looking for longer range and less selectivity, you may be interested in VLF “natural radio”.