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I see a lot of examples of simple science fair generators with the coil axis perpendicular to the rotational axis of the magnets. I have made one with the coil wrapped around the axis of rotation, but I cannot detect any output. (I wrapped a piece of PVC pipe with enameled wire and ran the axis of the magnets through the center of the pipe supported by a bearing salvaged from a hard drive.)

I'm not sure which of the following issues could be causing my failure:

  1. Not having the right LED (I have tried the LED in both directions)
  2. Not having enough wraps in my coil (I didn't count but used all the wire on the spool)
  3. Having the orientation wrong

Please advise.

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    \$\begingroup\$ From your description, I am pretty sure that the coil is not 'cutting' through the magnetic field, but is just 'in' the magnetic field which does nothing. However, a photograph or two should make it clear and unambiguous. \$\endgroup\$ – gbulmer May 15 '16 at 17:30
  • \$\begingroup\$ I have found instructions for a generator that seems to match what I have made. wikihow.com/Make-a-Simple-Electric-Generator \$\endgroup\$ – Bruno Bronosky May 16 '16 at 20:39
  • \$\begingroup\$ Please post some photo's or schematic diagrams of your generator. The generator at the link you posted isn't quite what I understood from your post. Are you saying the magnet is orientated exactly the same way? \$\endgroup\$ – gbulmer May 17 '16 at 0:41
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Yes.

Generators can be made with coils wound on an axis parallel to the generator axis, but they are not usually concentric with it. At one time they were known as "Ferranti dynamos", now "axial flux generator".

enter image description here

(Picture from the Granger Historical Archive Note : the generator is shown disassembled; the field coils (or permanent magnets nowadays) normally fit around the thin flat rotor.)

They often have many poles, to deliver reasonably good efficiency and relatively high frequency at a low rotational speed.

In fact the Ferranti dynamo shown here appears to be wound as a single continuous coil, and it is indeed concentric with the generator axis. The trick is that the convolutions on the rotor (centre of image) are aligned with the poles of the magnets ("easily removed for maintenance and cleaning" according to Ferranti's patent US409349) and thus cut the (axial) magnetic field, as required in JP314's answer.

Plans to build an axial flux generator are available here which will give you a good idea about them. The multiple coils in this version could be wound as a single continuous coil (probably 3 continuous coils as it is a 3-phase generator) to match the Ferranti design. But note that it inverts the Ferranti design shown above : the coil is stationary, and the permanent magnets surrounding it rotate.

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To generate an emf from the magnetic field, the coil has to 'cut' the field -- the total flux (magnetic field) through the coil has to change.

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