After reading the story behind the discovery of electromagnetic waves, I wanted to make my own coherer. The amazing thing about the coherer is how simple it is. Why it works is also mysterious. It seems the electromagnetic waves somehow magnetize the filings, and they get closer to each other. I built my coherer inside a vinyl tube with iron filings between two ¼” bolts.
I noticed that a very small amount of filings is required between the bolts for the coherer to work. Also, the performance of the coherer depends a lot on the gap between the bolts. I adjust the gap by first pushing the bolts until my multimeter (in continuity checking mode) beeps. Then, I pull the bolts out just a little until it stops beeping. That seems to be the perfect gap. Normally, the resistance between the two bolts will be very high – in megaohms. When the coherer detects electromagnetic waves, the resistance drops to 10-20 ohms. Normally the coherer will stay in this state of low resistance, and you have to tap it to decohere it. I am thinking about making an automatic tapping mechanism with an electromagnet.