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.

• iron fillings (you can either produce them yourself or buy them ready)
• 6x4 mm PU tube
• copper wire
• superglue
• multimeter
• piezo igniter (eg. a piezo lighter. I bought ad hoc one here)
• 3D printed part (see next step)

• 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!

Grade levels

• This instructable is well suited to a High School audience (9th-12th grade)

Pre-Req Knowledge

• A basic understanding of electricity, voltage, resistance and power is helpful.

Lesson Background and Concepts

About Waves

• First introduce students to the concepts of different types of waves and important features of waves. Upon understanding fundamental concepts about waves, discuss electromagnetic and radio waves more specifically.
• Introduce students to transverse and longitudinal waves, the two primary types of waves. A good visual for this can be found HERE. Ask students to follow the motion of a single particle so they can see that the particle oscillates in the same direction as the wave. Next show students a transverse wave. Individual wave particles for this type of wave move in a direction perpendicular to the direction of wave propagation. Thus, if a wave is propagating horizontally, the particle will be displaced vertically, moving up and down.
• Continue to explore transverse waves since this waveform is used in radio signal transmission. Draw a sinusoidal waveform on the board and identify the two major components: the signal's amplitude and the frequency (see Figure). Another important component of the wave is its frequency. Frequency is defined as the number of cycles a wave completes per second; it has units of hertz (Hz), where one Hz is equivalent to a 1/second. One cycle of a wave is the distance the wave travels until it reaches the same vertical position as where it started (see Figure).
• Next, explain electromagnetic waves and their relationship to AM radios. As current enters the antenna, a magnetic field is created around the antenna. Magnetic fields also induce an electric field in an antenna or wire placed close to the first wire. When no wire coil is close to a radio wave transmitter, the magnetic field around the antenna induces an electrical field in the open space surrounding it. In turn, this electric field creates another magnetic field in the space surrounding it. This change between electrical and magnetic fields propagates the wave through space, creating an electromagnetic wave.
• Note how a spark is an electrical current flowing without the presence of a physical antenna, but the behaviour is the same: a magnetic field is created around the spark, the magnetic field also induces an electric field, the electric field also induces a magnetic field... and so on.
• Introduce Gugliemo Marconi and its invention, describe the circuit and show the students where the coherer was located.
• Finally build the coherer and perform the demonstration with the students.