In 1902, Guglielmo Marconi received the letter S in Morse Code from across the Atlantic.
His receiver used a coherer as its detector and, since today is Marconi’s birthdate, I decided to to build one myself and make a video of its operation.
Although the coherer’s invention is often attributed to Marconi, it was actually invented by a Frenchman, Edouard Branly, but it was Marconi who put it to great effect and, as a result, became the world’s first DXer.
In the video, note that the LED stays lit even after the spark has ceased. First generation coherers such as this needed to be “de-coherred” with a tap. Second-gen coherers used a drop of mercury rather than iron filings which did away with that necessity and allowed a dot
to be distinguished from a dash.
And of course Marconi used a set of headphones rather than an LED – the received signal manifested itself as a brief static crash in the ‘phones regardless of whether a dot or a dash was sent.
This is the reason the letter S was chosen – an O would have sounded exactly the same (but would have allowed more time for de-coherring between characters).
Here’s my coherer in action: