Date: Sat Jul 14 10:39:27 2007

Author: Urs Lauterburg

Subject: Re: Van Allen Belt Demo


Sorry for the delay with answering. I am in the process of trying to
catch up with the incoming flow of mail (which even minus Spam has
increased to a soon to be unbearable volume) and found your earlier
inquiry about the asymmetrical circle path of electrons in the tube.
First of all I think that due to manufacturing differences (I believe
the tubes and their innards are still handmade) there are slight
differences that affect the electron beam geometry. It is therefore
suitable to slide around the socket within the Helmholz coils. In our
setups the given leeway for free positioning of the tubes within the
coils were enough to compensate any geometric differences.

As nice as your Van Allen Demo is, I would not want to get near the
tube with a strong permanent magnet. I have permanently wrecked tubes
by magnetizing the cathode/anode/aperture structure within the tube
due to a remaining magnetization. This was with an older type tube
from Tel-Atomic but I assume the same still applies to the present
models. Even inserting the faulty tube into a strong AC field did
never fully restore the original quality. We first wanted to hand out
permanent magnets to our students to experiment along the lines of
your Van Allen Belt model but for the reasons stated we did decided
against it. Your pictures made me wonder if it would not be this
which has changed the beam geometry.

Anyway, we actually get very good results of e/m with the apparatus,
but then again we would have the students to measure the diameters of
the circles with an aperture of a known diameter and a pinhole to
look through, all mounted along an optical bench perpendicular to the
electron path. This is all part of the student lab setup we made.

Nice pictures you made !

I hope this helps


Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern

>Good Day,
>I think I've created a new demo.
>Check this out: