The Begali Coherer is an ingenious demonstration of basic radio principles which can be used in a classroom for educational purposes, or just as a great conversation piece.
The “receiver” consists of a dipole antenna that is dimensioned for the 70cm band. It has 3 grams of nickel filings in a glass tube between both halves of the dipole. A 9V battery sends a current to a LED through the nickel filings.
Initially, and after each experiment, you would bump the receiver lightly, so that the nickel filings separate into a random (incoherent) pile. The electrical resistance of that configuration is relatively high, and the LED will not light.
The “transmitter” is an electronic lighter with a dipole antenna that is also dimensioned for the 70cm band. The high-voltage spark generated by pressing the button creates a broad band electromagnetic wave, and the dimension of the dipole antenna determines which part of that broad band spectrum is predominantly transmitted.
When the electromagnetic pulse reaches the receiving antenna, it will align the nickel filings into a coherent low-resistance configuration, and the LED will light up. This works well over a distance of up to 20 feet. You can also directly demonstrate the effect that the antenna polarization has: if the transmitting and the receiving antennas are oriented differently, the received signal will not be strong enough to bring the nickel filings into coherence.
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