The coherer was a primitive form of radio signal detector, invented by scientist Edouard Branly around 1890. It is nothing more than two electrodes placed one close to the other, with metal filings in the space between them. The two electrodes acts as an open circuit, until an EMP or RF energy is applied to it. The device acts then as a resistor. Youtube user gilbondfac made a replica of this device in his lab, and managed to transmit energy from a distance to light an LED. Watch his video.