Date: Thu, 20 Apr 2000 09:38:31 -0400 (EDT)

Author: Steve Wonnell

Subject: Re: Help! Cenco e/m experiment


Hi Doug!

When I taught at UNC-Chapel Hill, we used an apparatus very similar to the
CENCO 71267N, which is the model I think you refer to, with a horizontal
plate on which are inscribed concentric circles. When the electron beam
hits the circle, the spot where it hits fluoresces.

The apparatus worked great, but the main problem, according to the lab
manager at the time (late 1980's), was that the tubes "burned out" very
quickly and were expensive to replace. Current price is almost $700/bulb.
If memory serves me right, these tubes lasted about a year or so. What
went wrong essentially was that the beam grew too dim to be useful.

At Hopkins we use a different e/m model, also available from CENCO. This
is catalog number 32054N. This is the one where the beam enters the tube
horizontally with a wide horizontal spread and strikes a vertically
mounted screen that is oriented at a small angle with respect to the
beam's axis. The screen fluroesces. This apparatus has worked very well
even with heavy use (by 300 students each semester!). Our lab manager,
who's been here 20 years, says that these e/m tubes last indefinitely.
Usually the tubes don't burn out; what happens is that the students mess
around with the wiring, wire the tube up incorrectly, and burn it out. Our
concern with this apparatus is that it uses high voltage (as do all of
these tubes) and that the HV wiring is exposed. We put out these
apparatus already completely wired up, with a sign prominently displayed
saying "DANGER, HIGH VOLTAGE." No one (to my knowledge) has been shocked
yet. Despite the sign, every once in a while, some student gets the idea
that he or she needs to rewire the apparatus, and in doing so, sometimes
destroys the tube. In any case, the replacement tube for this apparatus
is only 8% ($50) more expensive than the apparatus that your folks are
considering purchasing, but lasts far longer. The apparatus is actually
made by Teltron LTD in England, not by CENCO. :)

Both models are easy to use and give good results (at least when
the beam of the 71267N is sufficiently intense, which is the case early
in its lifetime). A bit more calculation is involved with the 32054N,
in order to get the radius of curvature of the beam, whereas this value
is given directly with the 71267N. Nevertheless, I would not consider
purchasing the 71267N, because of the recurring high expense, unless
better e/m tubes are available. However, the 71267N relys on some kind of
vapor to illuminate the beam (the fluorescence of the concentric circles
alone, with the additional fluorescence of the illuminating vapor, is
too weak to determine the spot where the electron beam strikes). It is
likely that it is this vapor that degrades. The 32054N uses no vapor at
all. For this reason, I think that the 32054N is a much better buy,
as there is no vapor to degrade.

I use the Pasco type e/m apparatus (Pasco cat. no. SE9638) for use in
demonstrations. Because this apparatus doesn't see heavy use, and because
I don't use it for quantitative measurements, I can't directly compare it
with the other apparatus. However, certainly the CENCO 32054N must be
better for measurements, because the screen that the beam strikes in the
32054N has a scale, whereas there seems to be no way to directly measure
the the beam in the Pasco apparatus. With the Pasco, the beam is inside
a curved tube, so one can't be certain where the beam actually is, plus
there is the problem of parallax.

Good luck!

Steve W.

On Wed, 19 Apr 2000, Doug Johnson wrote:

> Date: Wed, 19 Apr 2000 14:18:31 -0700
> From: Doug Johnson
> Reply-To:
> To:
> Subject: Help! Cenco e/m experiment
> I need some comments on the Cenco e/m experiment. It is the one that bends
> the beam to hit some rings drawn on a flat plate inside of the tube. We
> are looking at replacing our Pasco type e/m apparatus with the Cenco one.
> The faculty are in favor of this change, but I think they will not be happy
> with it either. Any comments?
> If you could answer these questions, I would really appreciate it.
> Does it give good results?
> Is it easy to use?
> Are the tubes needing to be replaced often?
> Thanks in advance...Doug
> PS.. How many out there in Tap-L land use the Cenco type e/m apparatus?

Steven K. Wonnell
Physics and Astronomy Department E-Mail:
Johns Hopkins University Phone: (410) 516-4696
3400 N. Charles Street Fax: (410) 516-7239
Baltimore, MD 21218-2686 Office: 534 Bloomberg (x5468)