What is this box for? Coherent explanations are sought

Our member Jerry, M5JB, brought this handsome wooden box to our 50th anniversary special event station in Field Lane. It had belonged to his father, also a licensed amateur. But what was it?

Various guesses were made, but similar explanations were offered by Gerald, G3SDY, and Robert, G8BUN: It is – or was – a coherer. It turns our that the first radio transmissions were made using a spark transmitter and a receiver known as a 'Coherer'.

It was a primitive form of radio signal detector used in the late nineteenth and early twentieth centuries, consisting of a capsule of metal filings in the space between two electrodes.

In 1890 Edouard Branley found that if high frequency oscillating currents were passed through a series of metal fillings in a glass tube, the fillings tended to cohere and become more conducting. This was a key technology for radio, as it was the first device used to detect radio signals in practical spark gap transmitter wireless telegraphy.

Its operation is based upon the large resistance offered to the passage of electric current by loose metal filings being decreased under the influence of RF. Without the influence of the radio frequency, the fillings passed very little current.


The coherer became the basis for radio reception around 1900, and remained in widespread use for about ten years. It saw commercial use again in the mid 20th century in a few primitive radio-controlled toys that used spark-gap transmitter controllers.