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[Phys-L] electricity demo apparatus and radiation

What follows is an old discussion about the safety of Cathode Ray
Tubes when used in conjunction with the Leybold spark coil. The air
stopping, which John Denker mentioned was recognized in one of the
replies. I do not know what difference the use of the tesla may make.
Perhaps its not the high frequency due to the intermittent nature of
the device, but the far lesser current available due to the air gap.
I vaguely recall that the hand held tesla's were "safer". I may be
recalling incorrectly. Karl


If you want accurate measurements of low energy x-rays see if you can find a
Victoreen R-meter.

Also remember the issue isn't a one time use; what concerned me in my work
was that I would be setting up this equipment several times a year and the
accumulated dose. That said the risk is still not that great as risks go.
Being real close to the hottest tube I had for a minute would be like
smoking a pack of cigarettes riskwise.


----- Original Message -----
From: "Sam Sampere" <>
To: <>
Sent: Thursday, October 10, 2002 2:21 PM
Subject: Re: Cathode ray tubes for demos

I think you need to be more careful here. We all do for that matter.
we're the pros and we need to KNOW, not just think. I don't know, so I'm
forward to more decisive answers here. I don't think that "I don't
think" is good
enough when you're dealing with ionizing radiation. Make sense? It's our
health as
well as the faculty, students, etc. Whoever handles this rig should be

What evidence do you have for that statement? Have you measured the
voltage, emission current, etc? How about doing as Cliff suggested and
monitored the
x-rays? Guess what I'll be doing tomorrow?



"Jason St. John" wrote:

> I don't think that these old tubes accelerate (decelerate) enough
> electrons to generate appreciable X-ray radiation. Not enough current.
> -jmsj
> ____________________________________________________
> Jason St. John 617.353.2634
> Boston University Physics Lecture Demonstrations
> On 2002-10-10.14:49 sent:
> Would that include the old demo at this website?
> ----- Original Message -----
> From: Cliff Bettis
> Sent: Thursday, October 10, 2002 1:42 PM
> To:
> Subject: Re: Cathode ray tubes for demos
> The voltage you apply determines the energy, the current determines the
> intensity. You can limit the emission by keeping the voltage and current
> low as possible. The problem is that, traditionally, these tubes were
> off of induction coils and the only regulation was a spark gap in
> with the circuit.
> As far as detecting the radiation, a Geiger counter or film badge will
do. A
> friend of mine turned this issue into a class project using the tube to
> some x-ray photographs!
> Usually the audience is far enough away from the device that there is
> much risk for them, however, the lecturer or set-up person has to be up
> close. There is also the matter of the law and in our state we would
> the law if we operated an unlicensed x-ray source. So we generally use
> cathode ray tubes designed to run at low voltages (a few kV) with heated
> cathodes.
> I don't mean to alarm anyone, just to point out something you should be
> aware of if you operate these tubes. I, personally, wear a film badge if
> set up one.
> Incidentally, gas filled tubes (plasma globes and the like) don't emit
> x-rays because the electrons in them never get going fast enough before
> collide with a gas atom and have to start accelerating over again.
> Cliff
> ----- Original Message -----
> From: "Jim Kernohan" <Jim_Kernohan@Milton.Edu>
> To: <>
> Sent: Thursday, October 10, 2002 11:24 AM
> Subject: Re: Cathode ray tubes for demos
> > writes:
> > >
> > >Be careful when you use these old tubes as they can emit x-rays.
> >
> >
> >
> > UNder waht conditions will they emit x-rays? Should I shorten the
> > of time? lessen the voltage?
> >
> > How do I easily detect if I'm emiting x-rays?
> >
> > thanks!
> >
> > JimGet more from the Web. FREE MSN Explorer download :

The follow up reply from Jason St John was:

In fact, I checked it out just now, and I get 6 R/hr at the distances from
the tube where the instructor's hands typically are, and a solid 7 R/hr at
the end of the plate where most of the beam is stopped.


Dr. Karl I. Trappe, outreach consultant Home (512) 264-1616
Research Scientist Associate V (retired)
Senior Lecturer in Physics (retired)
Physics Department, Mail Stop C-1600
The University of Texas at Austin
Austin, Texas 78712-1081
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