Re: Bobs for Lenz's law apparatus

[Michael Bowen <fizzbowen@MINDSPRING.COM>]

At 09:21 2001/07/07, you wrote:
> ... It's very mysterious and
>  provides an excellent vehicle for explaining magnetic induction and
> Lenz's law.
>
> Our problem is that the little bobs have a tendency to disappear. I'm
>  sure our visitors don't steal them; they must oxidize into pure gase
> s in 24 to 48 hours. I called Sargent Welch to get replacement bobs b
> ut neither they nor their vendor sell only the bobs.
>
> I am soliciting ideas on how to replace the bobs. They are cylindrica=
> l, roughly 3/4-inch diameter and 2-inches long, are visually identica=
> l. If necessary, we could use non-cylindrical bobs as long as they ar=
> e visually identical and the correct size.

Our bobs don't disappear but they do tend to start falling apart after
multiple impacts with the floor. I seem to recall that the pieces (of the
magnetic bob) include aluminum and magnetic cylinders encased in some sort
of non-metallic cylindrical shell, but it has been about a year since I
last put the pieces back together, so I'll have to look into it again when
I get to campus. You might try one or two small cylindrical neodymium
magnets encased between short aluminum cylinders of the same diameter
(which you can manufacture in your machine shop if necessary), wrapped in
duct tape; the dummy can be a single aluminum cylinder of the same
composite size and wrapping. Get yourself an extra neodymium magnet and
drop it solo through the Al tube as you watch from above; it gyrates in an
almost "ghostly" manner, and makes the demo even more fun to watch. Also,
once you've revealed that one of the bobs is magnetic, be sure to show the
students that it doesn't attract the aluminum; otherwise many of the
students will think that it's a ferromagnetic effect (they quickly forget
that aluminum is non-ferrous).

Another fun demo is to get an old 5-MB hard drive from the junk pile that
is probably lying around your school's computer science or business
department and take the cover off so the platters are visible. Plug it into
a disused PC power supply, let it spin up, and then hold a strong magnet
near the spinning platters. (Eye protection might be a good idea here.) You
can hear the drive motor slow down in response to the interaction between
the eddy currents and the magnetic field; some drives have an auto-stop
mechanism that shuts them off if the rotation speed drops too low,
apparently to keep the motor from burning out. Again, be sure to show the
students that the direct attractive force between the magnet and
non-spinning platters (which are usually aluminum with a thin rust-like
coating) is negligible.