File:Blondel adjustable coherer tube (Rankin Kennedy, Electrical Installations, Vol V, 1903).jpg

English: Blondel adjustable coherer from a 1903 book. The coherer, invented in 1890 by Edouard Branly, was a primitive radio wave detector used in the first radio receivers during wireless telegraphy era around the turn of the 20th century. It consists of two metal electrodes C in a glass tube with a small quantity of loose metal filings between them. When a radio wave was applied to the electrodes it decreased the resistance of the filings, making a circuit between the two electrodes. An external circuit attached to the electrodes, powered by a battery, created a "click" sound or made a mark on a paper tape when the radio signal was present. The unique feature of this coherer design is that the quantity of metal filings between the electrodes, and thus the sensitivity of the device, could be adjusted by rotating the glass tube to add or remove filings from the reservoir P in the curved section.