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Picture of 3D Printed Coherer

French physicist Edouard Branly in the 1890s investigating the electrical characteristics of finely divided conductors such as metal filings, found that the overall resistance dropped sharply when electric sparks were generated in the vicinity. The original high resistance was restored by any slight mechanical disturbance of the tube. He drew on research performed in the mid-1880s by Temistocle Calzecchi-Onesti.

This phenomenon was employed as a detector of radio waves by Popoff, Tesla, Bose and Lodge. Lodge named the device "coherer" from the way in which the particles appeared to make better electrical contact when exposed to the high-frequency oscillating currents induced by radio waves.

Within a little more than a decade, Guglielmo Marconi would send the first wireless communication across the Atlantic with a radio device which used the coherer as part of the receiving unit.

Building and testing a simple coherer is matter of minutes and can be done with very simple materials and skills. Excluding 3D printing, the whole device will not require more than 10 minutes to be assembled and tested.

Step 1: Assembly and Test

  • Download the 3D models of the components required and print them.
  • Cut a length of 29mm of PU tube Fit one end of the tube in the 3D printed side support
  • Fit a piece of copper wire in hole on the side support. Make sure the length of the wire protruding into the PU tube is not more than 15 mm. I found out that 13mm work decently. Secure it in place with a drop of superglue.
  • Fit the 3D printed support on the 3D printed base. Secure it in place with a drop of superglue.
  • Fill the tube with iron filings.
  • Fit the second side support on the PU tube and then on the 3D printed base. Secure it in place with a drop of superglue.
  • Again fit the copper wire it in the hole on the side support. Secure it in place with a drop of superglue. Make sure the two copper wires are not touching.

Your coherer is ready!

Now set your multimeter to continuity mode (it should beep if you short the terminals) and connect it to the coherer wires. Generate a spark with the piezo igniter and notice the multimeter now beeping to indicate a closed circuit: the coherer is now conductive!

It detected the radio impulse generated by the spark. To reset the coherer you need to tap it lightly in order to disrupt the iron filings inside. Check the video and notice how tapping the device restore it to its original non conductive state. The device behave like a bistable switch: when it becomes conductive it stays conductive.

Coherence of particles by radio waves is an obscure phenomenon that is not well understood even today. Recent experiments with particle coherers seem to have confirmed the hypothesis that the particles cohere by a micro-weld phenomenon caused by radio frequency electricity flowing across the small contact area between particles.

The coherer was replaced in receivers by the simpler and more sensitive electrolytic and crystal detectors around 1907, and became obsolete. Anyway being cheap and easy to build and test, makes it a good STEM project with a lot of learning potential!

Wrrr 10-G2 months ago
Well done sir, a pleasure to read. I absolutely would enjoy reading more 'oldies' in hands-on physics experiments like these, it's very inspiring.
Have you thought of adding a mobile phone vibration motor to 'reset' the condenser on set intervals? That would be a funny contrast in technology ;)
hombremagnetico (author)  Wrrr 10-G2 months ago
Thank you sir! :) The mechanism to reset it is called a de-coherer and usually it was a solenoid. A vibration motor will do just as fine I think!
What was that high-pitched whirr in the video?
hombremagnetico (author)  Supernerd Sven2 months ago
Hi Sven, it's the multimeter's buzzer. After the spark the coherer is conductive and the multimeter's buzzer goes off.
Thank you!
ajoyraman2 months ago
Very well written, you have my vote!
I wonder if this concept could be used as part of a 'modulator' in a simple transmitter scheme?
hombremagnetico (author)  ajoyraman2 months ago
Thanks! :) What do you mean with "modulator"?
woodenhead3242 months ago
hi 40 or 50 years ago I had an electrical book that mentioned a coherer,tried at the time to find out what it did but to no avail (no internet then) thanks for the info, even made one back then but didn't have a thanks to you it is all clear. thank you!
hombremagnetico (author)  woodenhead3242 months ago
you're welcome! :)
JamesA412 months ago
Wondering if with a SMPS, if this would be a latching switch that you can tap to turn off? Seems like is and also wondering how small these can be made? Thanks for sharing!
hombremagnetico (author)  JamesA412 months ago
Hi James, the coherer is indeed a (cumbersome) latching switch. The dimensions could be lowered depending on the solution adopted. Anyway to get a reliable device you should experiment with filings size, wires distance at least. Other parameters influencing the behaviour are the filings composition (e.g. iron, zinc, copper or mixtures) and the use of a glass tube under vacuum for example...
mickeypop2 months ago
it should be noted that Tesla actually beat Marconi to the 1st transatlantic radio transmission and patents were also latter transferred.
hombremagnetico (author)  mickeypop2 months ago
Thanks for the tip!
JeromeS292 months ago
I'm goning to try this when I get back to the house today for sure
hombremagnetico (author)  JeromeS292 months ago
great! post your images of you do! :)
Alex in NZ2 months ago
Wow! I haven't come across a coherer in forever. Thank you for spreading the knowledge and sharing your work :-)
hombremagnetico (author)  Alex in NZ2 months ago
Thanks Alex. I'm glad you like it! :)