Re: Magnetic pendula

[Pete Lohstreter <plohstreter@MAIL.HOCKADAY.ORG>]

Forum for Physics Educators <PHYS-L@lists.nau.edu> writes:
> We have all seen those desktop novelties: battery powered pendula, a
> metallic circular frame rotating within a swinging circular frame.  Some
> look (kind of)like bohr atoms, some have a model space shuttles going
> around
> in apparently chaotic behavior.
>
> A battery powers an electromagnet in the base, which acts on a permanent
> magnet embedded in the pendulum.  Somehow this keeps the pendulum going
> despite frictional losses.
>
> Thinking about it has me wondering why the force from the electromagnet
> doesn't do as much slowing down of the pendulum as it does speeding up.
> Any ideas?

The e-magnet is not on all of the time.  There is a transistor in the
circuit that is triggered by the approach of the perm. mag. to the coil.
A short burst of current gives the perm. mag. a little kick every time it
passes the low point of the base.

Pete Lohstreter                         "Happy is he who gets to know
The Hockaday School               the reasons for things. "
11600 Welch Rd                        Virgil (70-19 BCE) Roman poet.
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