Hi all,
I built a Faraday disk to satisfy my curiosity. It is a disk of plate aluminium (approx 9mm thick and about 100mm radius).

I am getting a significant difference in the voltage I read from the disk depending on where I place the brush on the outer rim of the disk. If the brush is right under the magnets the voltage is at about 2mV. As I move the brush away from the magnets the voltage drops off quite quickly. More than about a few centimetres from the magnets it drops to 0V. All these measurements were done by hand turning the disk via an insulated crank handle at a speed of 60 RPM.

I was under the impression from reading that the voltage created by a Faraday disk is not localised to one particular segment of the disk.

Can anyone tell me whether what I'm seeing is expected?
Cheers,
Ben

 

Faraday disk circuit. The disk in the circuit acts as a low value bulk resistor for the current loop under the magnet as the disk turns. If we select the center of the disk for our 'ground/common' voltage reference and the outer edge as output voltage reference for the circuit you can see how the voltage will vary near the shaded area of the magnetic field if you move the pickup away from the magnetic field.

Now I’ve drawn the picture with the magnetic field pointing out of the screen; thus you see the “tip” of the field “arrow.” Using the right-hand rule, we can see that the wire currently passing through the magnetic field will experience a magnetic force that pushes a current from the axis to the outer rim of the wheel. If we replace our wire disk with a solid metal disk, the basic idea remains the same, although the current is free to spread out over a larger area of the disk as it travels from the center to the rim.

Read this and ask again if you have more questions.
https://skullsinthestars.com/2014/08/27/physics-demonstrations-faraday-disk/

Faraday treated motional emf and induction as being two aspects of the same phenomenon, though we have seen that they are evidently produced by different forces: motional emf comes from the magnetic field, while induction comes from an induced electric field. So Faraday was somewhat wrong in his analysis, but he was also accidentally right: in Einstein’s special theory of relativity, electric and magnetic fields are two aspects of the same electromagnetic phenomenon. The magnetic field seen by our stationary experimenter is perceived by the walking visitor as an electric field. Einstein’s first paper on special relativity in fact discussed in detail how magnetic and electric fields transform into one another when one changes to a different moving frame of reference. So, in fact, Faraday was also right in thinking of motional emf and induction as the same thing!

Faraday discovered something very important that took Einstein to explain correctly.

 

The voltage will be confined to the area under the magnet and nothing will be generated outside of the flux path.

 

Thanks, nsaspook and Glenn. That clarifies it for me. Thanks for the additional explanation and link as well, nsaspook