The Coherer - the simplest detector of radio waves?

 

Here's a simple circuit consisting of a power supply, an LED and series resistor, in series with a coherer made of some scrunched up aluminium foil balls inside a plastic beaker in contact with each other and a couple of aluminium foil strips wired to complete the circuit. 

Except that the circuit is only completed once there is a nearby source of radio waves, which somehow creates a low resistance pathway for the electrons in the aluminium balls. In demonstrations, this is usually achieved with a piezo-electric gas igniter. However, in this video, made perhaps 18 months ago, I instead touch one of the foil strips with a short length of wire which acts as an aerial to pick up radio waves from, IIRC, the electric motor of an adjacent freezer. The LED (top left) then lights up. If I tap the side of the beaker sharply, the pathway is disrupted and the LED turns off again.