Page "Wireless telegraphy" Paragraph 23

In 1890, Édouard Branly demonstrated what he later called the " radio-conductor ," which Lodge in 1893 named the coherer, the first sensitive device for detecting radio waves.

Shortly after the experiments of Hertz, Dr. Branly discovered that loose metal filings, which in a normal state have a high electrical resistance, lose this resistance in the presence of electric oscillations and become conductors of electricity.

This Branly showed by placing metal filings in a glass box or tube and making them part of an ordinary electric circuit.

According to the common explanation, when electric waves are set up in the neighborhood of this circuit, electromotive forces are generated in it which appear to make the filings move closer together, that is, to cohere, and thus their electrical resistance decreasesaccordingly, Sir Oliver Lodge termed this piece of apparatus a coherer.

Hence the receiving instrument, which may be a telegraph relay, that normally would not indicate any sign of current from the small battery, can be operated when electric oscillations are set up.

Prof. Branly further found that when the filings had once cohered, they retained their low resistance until shaken apart, for instance, by tapping on the tube.