Date: Thu, 10 Oct 2002 12:40:18 -0500

Author: "Cliff Bettis"

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 as
low as possible. The problem is that, traditionally, these tubes were run
off of induction coils and the only regulation was a spark gap in parallel
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 make
some x-ray photographs!

Usually the audience is far enough away from the device that there is not
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 violate
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

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 I
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 they
collide with a gas atom and have to start accelerating over again.


----- Original Message -----
From: "Jim Kernohan"
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 length
> of time? lessen the voltage?
> How do I easily detect if I'm emiting x-rays?
> thanks!
> Jim