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Hi All,

As long as you can see the ionized gas I think you are safe from X-Rays
with the HV at around 10 KV. If you start to see a green glow from the
glass where the cathode rays hit it you could have an X-Ray problem.

Anyone else want to comment on this?

John Bowditch
Exhibits Director
Ann Arbor Hands-On Museum

-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of Stuart Kohlhagen
Sent: Tuesday, August 07, 2007 9:50 AM
To: [log in to unmask]
Subject: Re: Crookes Tube as a floor exhibit? [SEC=UNCLASSIFIED]

ISEN-ASTC-L is a service of the Association of Science-Technology
Centers
Incorporated, a worldwide network of science museums and related
institutions.
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about 25 years ago Questacon had a related exhibit on the floor.
 
we had a 1 meter long  75 mm diam glass pipe, sealed at each end with
flat plate electrodes.
visitors could hit a button, and start the exhibit. fireing up a high
vacuum pump and a high voltage source( Can;t recall the exact voltage,
but I'll see if I can find any details, but probably 10-30KV) was then
applied across the electrodes. the exhibit had dials which displayed the
air pressure (vaccuum) and current being passed through the tube.
 
a blue/pink glow would start around one electrode, and as the vacuum
increased, the glow would extend, and then split up into stripes, as the
stripes would decrease in number but extened in depth, at very high
vacuum, the tube would black out. You could follow the changes in the
tube with the changes in the current. NOT as clear a demonstration of
crooke rays, but a nice display, and one that has a lot of depth. The
bands are a useful prompt to discuss the acceleration of the electrons,
and how they need a certain energy to interact and ionise the various
gases. 
 
This ran for about 15 years with occational servicing of the pump oil.
We haven;t run any longitudinal studies on its health effects.
 
And you can get lead doped glass to put it in a box if it proves
necessary.
 
cheers
Stuart Kohlhagen
Manager New Concepts
Questacon
 


Classification: UNCLASSIFIED
________________________________

From: Informal Science Education Network on behalf of John Bowditch
Sent: Tue 7/08/2007 11:22 PM
To: [log in to unmask]
Subject: Re: Crookes Tube as a floor exhibit?



ISEN-ASTC-L is a service of the Association of Science-Technology
Centers
Incorporated, a worldwide network of science museums and related
institutions.
************************************************************************
*****

Hi Allan,

Years ago there was such an exhibit at the Boston Museum of Science. The
tubes were acquired from Central Scientific and had a small phosphor
screen that was sort of elongated. It worked well except that the tube
screen did wear out over time.

Having said that, I'm sure this could be done fairly easily. Get in
touch with me off line as I could probably have the tube made for you.

John Bowditch
Exhibits Director
Ann Arbor Hands-On Museum

-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of Allan Ayres
Sent: Monday, August 06, 2007 6:50 PM
To: [log in to unmask]
Subject: Crookes Tube as a floor exhibit?

ISEN-ASTC-L is a service of the Association of Science-Technology
Centers
Incorporated, a worldwide network of science museums and related
institutions.
************************************************************************
*****

Hi everyone,

We're knocking around the idea of building an exhibit based on the
Crookes Tube -- you know, those physics-lab cathode ray tubes, like
this:

http://www.uvm.edu/~dahammon/museum/crookstubes.html

(Basically, we'd like visitors to be able to deflect a particle beam
with a magnet -- to be able to do on a small scale what Ernest
Lawrence did on a grand scale.)

A couple of questions:  Has anyone ever seen such an apparatus as a
standalone exhibit?  I've personally only seen them in physics labs,
and I haven't been able to find any references to hands-on exhibits
(as opposed to Crookes tubes in museum collections; the Web is lousy
with those...).

A related but more important question: Would this be safe?  I found a
paper from 1967 that seems to indicate that the X-ray emission from a
Crookes tube is pretty stunningly high, regardless of whether you're
measuring in front of the unit or off to the side.  Unfortunately,
the paper doesn't give details like the voltage they're driving the
tube with or other information like that, but I'm starting to get a
sinking feeling of "UNDER NO CIRCUMSTANCES can we do this on the
floor" (short of sealing the exhibit behind leaded glass or
something)....

I'd appreciate hearing about any experiences or lab safety knowledge
anyone would like to contribute!  Thanks much,

--Allan



--
~~

Allan Ayres
Exhibit Developer
Lawrence Hall of Science
University of California, Berkeley
Berkeley, CA 94720-5200
[log in to unmask]
510-642-1254

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