Effect of position of brush on a Faraday Disk

Discussion in 'Physics' started by five_volts, Jun 14, 2016.

  1. five_volts

    Thread Starter New Member

    Jul 31, 2015
    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?
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    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.
    Read this and ask again if you have more questions.

    Faraday discovered something very important that took Einstein to explain correctly.
    five_volts likes this.
  3. Glenn Holland

    Active Member

    Dec 26, 2014
    The voltage will be confined to the area under the magnet and nothing will be generated outside of the flux path.
    five_volts likes this.
  4. five_volts

    Thread Starter New Member

    Jul 31, 2015
    Thanks, nsaspook and Glenn. That clarifies it for me. Thanks for the additional explanation and link as well, nsaspook :)