In homopolar generator with corotating magnet and a copper disc, do brushes need to be stationary and be on the outer rim in order to read voltage and why?

simplified model

To visualize, this is the simplified version of experiment that I've performed as a hobby, recently.

I assumed that, due to movement of confined (to atoms) charges in copper in magnetic field the, there will be a small potential building up on the rim or the shaft depending on the direction of movement and orientation of magnets, and potentially big currents will flow under low resistance as proven in experiments (refer to Faraday paradox).

The main purpose of this configuration is to be able to make use of the same rotational direction and opposing magnetic flux dirrections to (in addition) double the effect and (primarily) focus the voltage in the area of inner (just to point out the inner factor) shaft as the linear velocities are lower around the center of the configuration.

If you see a problem with that configuration, straight away here, you don't need to read further, and can answer now, why this can or can't work.

For all other I'll continue to explain my experiment.

Because I didn't have enough funds to perform all the intermediate steps and because I thought that I understand the nature of this phenomenon (shame on me), I thought that I can multiply the efect, the voltage, and efficiency (that is limit friction) by performing multiple passes arround the circle and alternately introduce magnets set into opposing poles on the way out to the rim, and the returning path.

Time for some illustrations:

Mask of the pcb for etching [Mask of the pcb for etching]

(wew. - wewnętrzne = inner) (zew. - zewnętrzne = outer)

(green is for routing out, black for copper to stay, white to get rid of in the process)

I have designed a pcb board where the mechanical shaft does not touch any part of the circut, instead some plumbing pipes connectors made out of brass are soldered centrically to the board (outer fi 25mm for those who want to try out). I will refer to them as outer shafts.

As you can see there are multiple paths going out towards the rim, and as many going back towars the shaft. After mutliple round the voltage should travel to the hole where it travels to the other site of the board that is the oposite of reflection image of the board on primary side so that the charge continues to in the same linear direction and find itself in the field of the same magnet as the first one going straight out of the brass outer shaft and then do the same circulation around as on the primary side of the board (with the difference that if will go through the outer circuit path first to the inner circuit path and direct to the inner side of the board hitting the out shaft on the other site of the board).

Depiction of connections between both sides of the board [Depiction of connections between both sides of the board]

On both sides of the pcb there is a circle made of plexiglas with placeholders for magnets and and outer full circle to secure the whole construction and allow me to drill wholes to put long screws through the whole construction and secure everything together with 3mm screws and nuts.

Cut form for magnets

The forms and the magnets are placed of both sides of the pcb in a way that magnets on both sides attract each other in facing pairs. The purpose of this is to create as strong and straight lines of magnetic flux as possible. The opposing direction of neighboring magnets is to (by what it seems a wrong assumption) to build up voltage on the way out to the rim and also build up the voltage on the way down to the shaft, and in this way, to multiply the effect.

Thank you very much for reading this up to this point. That fact is that, contrary to my simple experiment with a solid plate in magnetic field wher probes are: one on the shaft, one on the rim (where I registered 30 milivolts at 1500 rotations per minute), in this case I have registered no voltage.

Please be kind with me and if you know what did I do wrong, and hopefully, if you know what can be done to pass over my errors and keep the results of multiplied voltage, please explain in layman terms using thumbs, analogies what is wrong with this setup, and what can be done to make it work.

Disclaimer: Nowadays science is very hermetic for me, and even though I can visualize electron clouds of probability, how magnetism can bend the shape of them, how it arranges the spin of particles, vector cross product, I get lost on second dependent equation with vector integrals. So if you'd try, I appreciate it with all my heart.

I hope for a good discussion.

I hope that I don't break rules but let me add that.. Also, if you have an innovatory idea in this topic and want to cooperate with me, or you want to track my progress or if you have ides for me how to fix or improve my designs, want to work together, contact me.

  • $\begingroup$ I'm calling @HolgerFiedler as he/she has answered a related question in past. $\endgroup$
    – doker
    Jan 24, 2018 at 21:16

2 Answers 2


I would need better (3D) pictures to really grasp your device, but it seems that the reason you obtained some voltage on the first device is because of the copper lines going to the shaft (not from the rim).
The problem with the pcb device, is that the voltage induced on the out-going part, is cancelled by an equal and opposing voltage on the in-going part of copper loops.
The magnets orientation could also be a factor.

  • $\begingroup$ I think that you understand well the design. Magnets are on both side of the board and the magnets reach only as far as the ourgoing and ingoing straight paths. So the peripheral-round paths are outside of the magnets so my only idea is that the lines of field returning back just on the edges of magnets opose the primary field and create a strong resistance. That is why I ordered iron circular plates to cover the magnets on the outside and close their returning fields in the iron plates, and in the same time strenghthen the field that passes through the pcb to the other side. Soon will know. $\endgroup$
    – doker
    Feb 4, 2018 at 16:52
  • $\begingroup$ Moreover, if somene in future reads this, the reason why the generator works with static brushes is that although there is an opposing field on the edge of the magnet when the paths leaves it, the lorenz rule ( = original hall effect), does not apply because the conductor does not move, so from this place it does not care about the magnetic field, nomatter how strong it is. $\endgroup$
    – doker
    Feb 4, 2018 at 16:56

I am no expert. But...

I think the logic works, using the right hand rule, all of your forces add up if you ran a current through it.

A suggestion: You should try connecting all of the peripheral and axial traces, (that are side by side), together with a low resistance but removable connection to give you essentially a one wire low resistance path and see if that works at all. This will test the alignment of the currents and magnets, if it works at all, then likely you need thicker traces. Aluminum and an exacto knife with your current magnet setup could get you the one wire test for very little extra expense.

You might look at what Tesla said about subdividing a disc. I seem to recall he had a reason why you needed a spiral, and not straight lines.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.