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Suppose that the peak voltage measured on a solenoid when a magnet moves inside (back and forth, periodically) is V.

If we repeat exactly the same experiment but placing the solenoid inside an iron pipe (of slightly larger diameter), would this peak voltage increase, decrease or stay the same?

Thanks!

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It depends. If the magnet is fully immersed in the relatively uniform field inside the coil, you won't get much voltage. The positive and negative voltages induced by the changing fields will roughly cancel.

If just one pole (say, North) of the magnet is inside the coil, then its a different story, you can get large voltages. Putting a highly permeable iron sleeve around the coil will increase the fields and increase the voltage. Extending the iron sleeve back to encompass the other pole (South) of the magnet will help a lot more. This greatly reduces the reluctance for the magnetic flux returning from the North pole to the South pole. Finally putting another coil (with opposite winding polarity) around the South pole and putting the two coils in series will double the voltage again!

coil magnet iron configuration

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  • $\begingroup$ Hi @Roger Wood, this is exactly the kind of answer I was looking for, thanks a lot! Just as a curiosity, do you know if there's a similar thing that could be done if the magnet was magnetised in the opposite direction (N at the top half and S at the bottom half), and rotating around the horizontal axis instead of moving back and forth? $\endgroup$
    – Marcos
    Commented Jan 30, 2022 at 0:27
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    $\begingroup$ @Marcos Not with a solenoidal coil like that. The changes in field would be in the same direction as the wire in the coil. They are best at right angles. If you put a pancake coil above the magnet and another one below, then you'd have a single-phase en.wikipedia.org/wiki/Permanent_magnet_synchronous_generator $\endgroup$
    – Roger Wood
    Commented Jan 30, 2022 at 2:22

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