[Ashish] let us know about his experiments in recreating the earliest type of radio set: a spark-gap transmitter and iron-filings coherer. He goes through the historical development of the kit in great detail, so we’re just going to skip that part. Go read it yourself!
Instead, we’re going to tease you with the coolest part of the rig: the coherer. In [Ashish]’s build, it’s a piece of tubing with some iron filings between two bolts. When a sufficiently strong EM wave hits the filings, they stick together and bridge the gap between the bolts, allowing electricity to flow and light up an LED, for instance. You can see this in [Ashish]’s video below the break, along with kmore discussion of that coherer.
A coherer is a one-shot receiver — the filings have to be physically separated after each reception. Repeatedly tapping on the coherer by hand must have gotten old pretty quickly: period coherers included a “decoherer” — an electromechanical tapper that reset the coherer multiple times per second by hitting it, and contribute to a low buzzing sound when receiving with one.
Human ingenuity being what it is, there were many advances in coherer design over a few decades before crystals made them entirely obsolete. We love peering into these technological cul de sacs and finding that they were full of cleverness. We hope [Ashish] keeps playing around with coherers.
Most of us at Hackaday are licensed amateur radio operators, and we should definitely note that a spark-gap is a ridiculously broadband emitter, and you’re probably transmitting on all sorts of frequencies for which you don’t have a license. The fact that [Ashish]’s signal is strong enough to move iron filings across the room suggests that he might also be interfering with other people’s radio in the neighborhood, or further. A quick check with an AM radio, for instance, would be indicative.
It does look like it’s possible to get permission to run an experimental spark gap transmitter in the form of a Special Temporary Authority from the FCC, though, so all is not lost. If anyone else knows something about legally operating a spark-gap transmitter, please post up in the comments. The retro tech is cool enough that you know people are going to be trying this — let’s see if we can find a way to do so responsibly?
[Ashish] suggested to us that a coherer can probably make a pretty decent lightning detector. We know what our next quickie project is going to be.