MadSci Network: Other

Re: Why does putting a magnet on a tv cause the color to go to the middle?

Date: Wed Nov 1 17:21:01 2000
Posted By: Chris Seaman, Staff, Electrical Engineering, Materials Engineering, Alcoa Technical Center
Area of science: Other
ID: 972707325.Ot

A TV "picture tube" and a computer display screen are both "cathode ray 
tubes" or CRT's.

Dear Pam,

The display surface of the tube is coated with a phosphoresecent material, 
similar to that used in "glow in the dark" stickers or paints.  At the back 
end of the CRT a beam of electrons is shot towards the front.  These 
electrons make the front of the tube glow everywhere they strike it.

These electrons can be steered with magnetic fields created by 
electromagnets.  In fact, that is what is being done by the "bump" in the 
back of your TV.  The electromagnets sweep the beam back and forth and up 
and down to cover the entire front surface of the TV.  If you put a very 
strong magnet at the front of the TV, this field will also steer the beam.  
The typical effect is that the picture is distorted.

You can "magnetize" part of the picture tube in the same way that you can 
use a magnet to "magnetize" a nail or a screw driver.  So you can mess up a 
TV, but it requires a pretty strong magnet.  You can experiment with a 
refrigerator magnet to mess the picture up a little bit to see how it 
works.  This shouldn't hurt the TV.  If it does, tell your mom that your 
brother did it.

When computer displays show the same image over and over again, they can 
magnetize themselves.  This will cause the image to be distorted, sometimes 
messing up the colors.  For this reason, most computer displays come with a 
feature called "de-gaussing".  De-gaussing applies a very strong, quickly 
changing magnetic field to "scramble" any magnetized parts of the screen.  
This degaussing will also scramble cassette tapes and floppy discs.

Chris Seaman
Alcoa Technical Center

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