My video tribute to Marconi…and Branly

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

Marconi's coherer attached to an ivory rod. Click (and then click again) for larger image.

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:



  5 comments for “My video tribute to Marconi…and Branly

  1. April 25, 2012 at 2:07 pm

    Beautiful! Well done. What a nice minimalistic setup.

  2. Elwood Downey, WB0OEW
    April 26, 2012 at 12:15 am

    Excellent, thank you. It’s hard to appreciate now how he could even imagine to try that thing over several thousand miles. Also, I never knew that dit and dah would be indistinguishable on that first radio set.

  3. April 26, 2012 at 5:23 am

    Thanks for the comments. Yes, it’s easy for us to reverse-engineer the thinking of how this works but for the concept of radio to be a clean slate and be able to come up with this idea is just amazing. It makes me wonder what other ideas were tried that didn’t work.

    As far as what he heard, there was simply a very brief static crash as the coherer went from non-conduction to conduction. Dots and dashes were distinguished from each other by the dead time between characters, just as railroad telegraphers did with their sounders clacking away.

  4. Bob W0AO
    April 27, 2012 at 10:19 am

    There is an excellent book at the Internet Archive that has a lot of info
    about building transmitters and coherers. The first edition was in 1905
    and this edition is from 1913. Starting on the original page 10 you will
    see a drawing of a coherer and on page 12 the author tells you to start
    with a clean, coarse file, a nickel five cent piece, and a silver dime.
    The code was copied with a morse register which traced dits and dahs on a
    moving paper. The register was very expensive, so the author tells how to
    wind a relay so the code can be copied with a sounder. Lots of great
    reading and excellent diagrams. Find the book here:

    • April 27, 2012 at 10:24 am

      Thanks Bob – I’ll check that out.

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