The Coherer Effect 

The coherer effect is a physical phenomenon that was used in early radio receivers to detect radio waves. It was discovered by French physicist Édouard Branly in 1890.

A coherer is a simple device consisting of two electrodes separated by a small gap filled with loose metal filings. When a radio wave is applied to the electrodes, it causes the metal filings to stick together, or cohere. This reduces the resistance of the coherer, allowing a current to flow.

To use a coherer in a radio receiver, the coherer is connected in series with a battery and a bell or buzzer. When a radio wave is received, the coherer conducts, allowing current to flow through the circuit and ringing the bell or buzzer.

The coherer effect was used in early radio receivers, such as those used by Guglielmo Marconi to make the first transatlantic wireless transmission in 1901. However, the coherer was not a very sensitive detector, and it was eventually replaced by more sensitive and reliable devices, such as the crystal diode and the vacuum tube.