Date: Tue, 12 Sep 2000 15:08:00 -0500

Author: "Dennis C. Henry"

Subject: Re: crookes tube & CRTs


As I demonstrated at the 1994 summer AAPT meetings at Notre Dame, one can
do a variety of magnetic field visualization demonstrations around, and
particularly in front of a color CRT. I've never magnetized a shadow
mask so strongly that one of my "degaussing weapons" wouldn't clear it
completely. Pistol-type soldering guns, such as the Weller models,
produce a magnetic field in the range of 70-80 gauss (rms) nearby, and
hand-held or desktop audio and video magnetic tape demagnetizers reach
nearly half a kilogauss up close. I've had TV technicians (of whom I was
one myself years ago) express concern about warping the shadow mask, but
it's under enormous tension, and abused pretty badly by the gun beam,
already. The ferromagnetic character of the mask is a byproduct of the
engineering choices made to give it the other qualities needed for good
performance. I'm working (slowly) on a manuscript for
TPT on these and related topics.

A student and I just completed a year-long study to characterize and
identify the sources of DC magnetic fields in one of our classrooms that
produce color distortions on a video monitor placed in certain spots.
The effects are really dramatic. Stay tuned, as they say.


At 11:23 AM 9/12/2000 -0700, you wrote:

>Old tube (in addition to the kinescope, duh) o'scopes are a good
substitute (a reason for not throwing them out). Monochrome TV also --
As I've already posted (elsewhere) the only danger of using a

>magnet near the screen of a colour kinescope is one permanently
magnetizes the shadow mask (beyond the ability of the "degausser" to
"degauss.") I did this once and used a very large e-m speaker coil

>using the house supply to "degauss" the mask.


>Would someone tell me why they are made of ferromagnetic material? (I
have some guesses, but am not confident.)





>P.s. I suspect removing a colour TV (especially a large one) from its
case to demo v X B is dangerous for the Crooks tube reasons. The face (I
presume) is thick to shield viewers from the 30 + kV

>x-rays and protect in the event of breakage.

Dennis C. Henry Office Location: 213 Olin Hall

Professor of Physics office phone: (507) 933-7314

Gustavus Adolphus College office fax: (507) 933-6104

St. Peter, MN 56082-1498 home phone: (507) 931-2784